Had a thought while we were on a short tour,on our singles, this past week. Since we were riding to DC on the GAP & C&O I wondering if anyone has converted their tandem to a 650b wheel? We have a Co-Motion Speedster with disc brakes.
Would appreciate hearing about it!

We have 2 sets of wheels for my travel tandem one 650b with 38mm tires and one 700c with 28mm tires. I use the 650b for touring, gravel grinding, or climbing mountain passes. They are much more stable and comfortable. The 700c is for street or fast group rides.
Switching between the two with disc brakes is trivial just make sure you have the same disc size and spacing and the same cassette (unless you don't mind changing chains).
Be aware that the smaller 650b wheel will give you an extra lower gear and one less high gear which is great for their purpose.

A nice little write up...
Wheel Comparison: 650B vs 700c -

while YMMV, my experience has been very positive. When the rear wheel on our Cannondale started showing sidewall cracking I decided I wanted a spare wheel set. As such I picked up 4 velocity aileron rims, 2 32 hole 650b and 2 40 hole 700c (the bike came with 40 hole white hubs that I intend to re-use) We built up the 650b set first with a white mi6 xi hub for rear and a son 28 dynamo hub for front. The bike is so much more comfortable now that I have not built the 700c set . Currently running schwalbe marathon evolution supremes in a 42mm at 55psi. I ride with several different stokers and they have deemed my aluminium frame/fork cannondale as "the Cadillac"

I'm in the process of converting my 1997 Trek T200 at the moment. It has a Co-Motion Speedster Disc fork, so the front end of our bike is just like yours. In the rear, we have a canti brake for 700c rims. I'll be using a BMX U-brake mounted to the brake bridge for the 650b wheels. Ideally, I'll have enough adjustment on the rear brake pad slots to switch wheels without much modification.

Our 700c wheels are White Industries MI6 40h front, Shimano FH-HF07 48h rear, Velocity Dyad rims, DT Swiss Competition spokes. We typically use 35-38mm (measured width) road tires.
The 650b wheels will be DT Swiss Onyx 36h front, Velocity Blunt 35 front rim, Velocity Cliffhanger rear rim, Sapim Force spokes. Still deciding on rear hub (Shimano XT or Velocity tandem) and hole count (36 or 40). We'll be trying out Schwalbe Thunder Burt 27.5 x 2.1 tires first.