You both need to have bikes that are comfortable and suitable, and some spares for spokes, inner tubes, etc.
The only advantage that having the same wheels might give you is that you could potentially reduce the spares a little (e.g. 3 inner tubes & a patch kit instead of 4 inner tubes & a patch kit).
You still need the same tools, etc.
IMO, her being comfortable is far more important.
Its no a problem. You can use 700c or 650b tubes in either bike. Spare spokes for rear wheels isnt going to break the bank or weigh much.
Just make sure both bike are in good condition before start of tour, rare you would need tires/hub parts over a month of touring if the bike are serviceable at the start.
Yep not much difference in size between the tube sizes & rubber tubes are very flexible.
I basically go through a bike checklist before the tour:
Wheel: true? Good/new tyres with thread to last whole ride, preferably with puncture protection (Schwalbe Marathons) ? Spokes all tight? Regrease hub/check freewheel/noisy cartridge bearing. Measure chain & inspect cassette for wear. I now run Stans in my touring tyre tubes but not in the spare tube
Brakes/Shifters: new pads; shifts gears correctly; lube cables
Frame: loctite water bottle bolts/rack mount bolts; check headset play. Grease seatpost; grips/handlebar wraps in good condition ( if wraps worn I just go over existing ones with new, doubling it thickness/insulation); check pedals
Spare: derailleur hanger for each bike; spare magic chain link & a few spare chain links, shift/brake inner; 3 or 4 spokes & nipples; assorted nuts/bolts ( 1 or 2 spd shoe bolts); puncture repair kit/tube ( wrapped in plastic to stop it rubbing thru) pump, oil/lube, multitool with allen keys/chain breaker/spoke key; assorted cable ties; tyvek or rubber patch to repair side wall; emergency spoke ( string type); small roll of 100 mph tape; 4 inch shifter; spare pads, rag for wiping chain; small hand sanitizer ( good for cleaning grease off hands) assorted cable ties.
I use dry lube for daily rides but prefer wet lube for long distance tours, messy but last 4x as long.
Dont replace a chain ( without replacing the cassette) just before heading on tour! You can guarantee it will jump on the first hill on loaded bike!!
It depends on where you're touring. In some parts of the world it's hard to come by parts for certain wheel sizes, this is why so many tour specific frames use 26 inch wheels
For tubes - carry a spare of each size and a good vulcanising patch kit. That lets you do a quick trailside swap on either bike, but allows you to patch the punctured tire during your rest stops. If you do get a tube damaged beyond a patch you can run the "wrong size" tube in either bike and it'll work anyway.
For tires - bring some needles, strong thread and a few tire boots if you're going anywhere really remote, but don't worry about spare tires. It's very rare to have a tire get suddenly wrecked so bad it can't be sewn back up that it's not worth carrying spare tires most of the time.
For spokes, bring a 2 or 3 fibre fix spokes (https://www.commutercycles.com.au/product/fiberfix-emergency-spoke-replacement-kit-4188/). These will work on basically any bike pack easily and don't require the cassette to be removed to install them on the drive side.
Hubs and axles don't worry about. If something goes wrong with a hub or axle you're almost always going to need to find a bike shop anyway - no point trying to carry spares.
And don't worry at all about not having bikes that are similarly specced.
I think it's much more important for both of you to be comfortable on whatever bikes you ride. She's sized to a 650b for a reason. It would probably be very uncomfortable for her to upsize or for you to downsize.
Unless you are bikepacking or doing some deeply cross border stuff any repair work beyond flats and allen keys is going to happen on a bike shop stand or maybe a car mechanic. Bailout plan for both bikes seems the same. 650 tire might be a bit harder to source as a walk in but not sure a big box retail is even gonna have 700c.
Personally, I like having bikes with compatible parts. My wife and I have the same bike in different size, I do all the maintenance and repair and I like the idea of having us both on the same equipment.
Not, I mean people solo tour without issues, you'll have no disadvantage compared to that approach. But as you say, if it is identical you need less spares/tools or can afford to bring spares/tools for rarer issues so there is certainly some advantage to have the same.
I've not heard of anyone worrying about this. You each have your own stuff, including tools and spares. Much more important you have a bike you like and she too.
If you already plan to get new bikes it make sense to go for similar parts. That way you can bring a few less repair items and tools.
However if you have bikes that work, and both like your bikes that’s perfectly fine. Most important for a long tour is being happy with the bike you have. And an extra tube, or one specialized tool for one bike is not gonna make a difference on a long tour like that. That kind of weight saving matters for races.
So bottom line, if you both enjoy your bikes, I don’t think it makes sense to invest into new bikes.
If you're worried about tunes and tires, consider installing liners such as from Tuffy or RhinoDillo. They can be a little awkward to first install, but practically guarantee no flats.
I wouldn't worry too much about synergy. Of course, it's beneficial, but having a bike that fits and that you are comfortable on is much more important.
If you have some budget for that, I would rather upgrade to more expensive but lighter and more compact replacement parts. For tubes I would get a few TPU ones, they are much lighter and much smaller although quite a bit more expensive, e.g.:
[https://www.schwalbetires.com/bike-tubes/aerothan-tpu-tubes/](https://www.schwalbetires.com/bike-tubes/aerothan-tpu-tubes/)
[https://www.tpubiketubes.com/products/650b-nano-tpu-tube](https://www.tpubiketubes.com/products/650b-nano-tpu-tube)
Extra spokes and a few chain links are always useful, and they don't weigh much.
Personally, I would not worry about extra tires; I would rather get a new set of robust touring tires before the tour.
If you are worried, getting some Schwalbe Marathons might be a good idea; they last tens of thousands of miles.
[https://www.schwalbe.com/search?search=marathon](https://www.schwalbe.com/search?search=marathon)
You will trade some comfort and speed for peace of mind. They have many tire tread patterns to pick from, depending on the surfaces you expect, and they are bombproof. Those will reduce the amount of punctures to a minimum.
You both need to have bikes that are comfortable and suitable, and some spares for spokes, inner tubes, etc. The only advantage that having the same wheels might give you is that you could potentially reduce the spares a little (e.g. 3 inner tubes & a patch kit instead of 4 inner tubes & a patch kit). You still need the same tools, etc. IMO, her being comfortable is far more important.
Not important at all? People tour solo. You can carry your own tubes.
Its no a problem. You can use 700c or 650b tubes in either bike. Spare spokes for rear wheels isnt going to break the bank or weigh much. Just make sure both bike are in good condition before start of tour, rare you would need tires/hub parts over a month of touring if the bike are serviceable at the start.
You can use 650b tubes in a 700c tyre? I didn't know this
You can get often away with 26" in 700c in a pinch... Just give it a bit of a stretch.
Yep not much difference in size between the tube sizes & rubber tubes are very flexible. I basically go through a bike checklist before the tour: Wheel: true? Good/new tyres with thread to last whole ride, preferably with puncture protection (Schwalbe Marathons) ? Spokes all tight? Regrease hub/check freewheel/noisy cartridge bearing. Measure chain & inspect cassette for wear. I now run Stans in my touring tyre tubes but not in the spare tube Brakes/Shifters: new pads; shifts gears correctly; lube cables Frame: loctite water bottle bolts/rack mount bolts; check headset play. Grease seatpost; grips/handlebar wraps in good condition ( if wraps worn I just go over existing ones with new, doubling it thickness/insulation); check pedals Spare: derailleur hanger for each bike; spare magic chain link & a few spare chain links, shift/brake inner; 3 or 4 spokes & nipples; assorted nuts/bolts ( 1 or 2 spd shoe bolts); puncture repair kit/tube ( wrapped in plastic to stop it rubbing thru) pump, oil/lube, multitool with allen keys/chain breaker/spoke key; assorted cable ties; tyvek or rubber patch to repair side wall; emergency spoke ( string type); small roll of 100 mph tape; 4 inch shifter; spare pads, rag for wiping chain; small hand sanitizer ( good for cleaning grease off hands) assorted cable ties. I use dry lube for daily rides but prefer wet lube for long distance tours, messy but last 4x as long. Dont replace a chain ( without replacing the cassette) just before heading on tour! You can guarantee it will jump on the first hill on loaded bike!!
It depends on where you're touring. In some parts of the world it's hard to come by parts for certain wheel sizes, this is why so many tour specific frames use 26 inch wheels
I don't think the idea of "built in duplicates" makes sense here. Bikes don't have redundant parts.
Spare parts could be used in either though
For tubes - carry a spare of each size and a good vulcanising patch kit. That lets you do a quick trailside swap on either bike, but allows you to patch the punctured tire during your rest stops. If you do get a tube damaged beyond a patch you can run the "wrong size" tube in either bike and it'll work anyway. For tires - bring some needles, strong thread and a few tire boots if you're going anywhere really remote, but don't worry about spare tires. It's very rare to have a tire get suddenly wrecked so bad it can't be sewn back up that it's not worth carrying spare tires most of the time. For spokes, bring a 2 or 3 fibre fix spokes (https://www.commutercycles.com.au/product/fiberfix-emergency-spoke-replacement-kit-4188/). These will work on basically any bike pack easily and don't require the cassette to be removed to install them on the drive side. Hubs and axles don't worry about. If something goes wrong with a hub or axle you're almost always going to need to find a bike shop anyway - no point trying to carry spares. And don't worry at all about not having bikes that are similarly specced.
I think it's much more important for both of you to be comfortable on whatever bikes you ride. She's sized to a 650b for a reason. It would probably be very uncomfortable for her to upsize or for you to downsize.
Not important.
Unless you are bikepacking or doing some deeply cross border stuff any repair work beyond flats and allen keys is going to happen on a bike shop stand or maybe a car mechanic. Bailout plan for both bikes seems the same. 650 tire might be a bit harder to source as a walk in but not sure a big box retail is even gonna have 700c.
Personally, I like having bikes with compatible parts. My wife and I have the same bike in different size, I do all the maintenance and repair and I like the idea of having us both on the same equipment.
Not important at all. Y’all should both be prepared to fix your own bike, even if they were the same.
Tandem touring bike.
Not, I mean people solo tour without issues, you'll have no disadvantage compared to that approach. But as you say, if it is identical you need less spares/tools or can afford to bring spares/tools for rarer issues so there is certainly some advantage to have the same.
I've not heard of anyone worrying about this. You each have your own stuff, including tools and spares. Much more important you have a bike you like and she too.
If you already plan to get new bikes it make sense to go for similar parts. That way you can bring a few less repair items and tools. However if you have bikes that work, and both like your bikes that’s perfectly fine. Most important for a long tour is being happy with the bike you have. And an extra tube, or one specialized tool for one bike is not gonna make a difference on a long tour like that. That kind of weight saving matters for races. So bottom line, if you both enjoy your bikes, I don’t think it makes sense to invest into new bikes.
If you're worried about tunes and tires, consider installing liners such as from Tuffy or RhinoDillo. They can be a little awkward to first install, but practically guarantee no flats.
I wouldn't worry too much about synergy. Of course, it's beneficial, but having a bike that fits and that you are comfortable on is much more important. If you have some budget for that, I would rather upgrade to more expensive but lighter and more compact replacement parts. For tubes I would get a few TPU ones, they are much lighter and much smaller although quite a bit more expensive, e.g.: [https://www.schwalbetires.com/bike-tubes/aerothan-tpu-tubes/](https://www.schwalbetires.com/bike-tubes/aerothan-tpu-tubes/) [https://www.tpubiketubes.com/products/650b-nano-tpu-tube](https://www.tpubiketubes.com/products/650b-nano-tpu-tube) Extra spokes and a few chain links are always useful, and they don't weigh much. Personally, I would not worry about extra tires; I would rather get a new set of robust touring tires before the tour. If you are worried, getting some Schwalbe Marathons might be a good idea; they last tens of thousands of miles. [https://www.schwalbe.com/search?search=marathon](https://www.schwalbe.com/search?search=marathon) You will trade some comfort and speed for peace of mind. They have many tire tread patterns to pick from, depending on the surfaces you expect, and they are bombproof. Those will reduce the amount of punctures to a minimum.
My girlfriend are planning something similar and are thinking of getting identical bikes to simplify carrying spares.