I think a cost-effective way to build or buy a tandem that will accept high-volume tires is to get a frame designed for 26” (559) wheels. You can certainly build it up as a road/gravel bike with drop bars. On our da Vinci Joint Venture road style tandem, we run Schwalbe Marathon Supreme 26 x 2.0 tires and still have room for fenders if we want. Some quick math puts the tire diameter at 25.98” vs. a 700c x 25 tire at 26.45” so there is only about 1/2 inch difference in tire diameter from what the OP is using now, so the wheels won’t look oddly small.
Our all-up weight with 6 full large bottles and a full large rack trunk is around 420 pounds. We run 50/56psi front/rear and these tires roll fast on pavement but really soak up road vibrations and bumps. There are still plenty of 26” tire choices from Rene Herse, Schwalbe, Panaracer, Kenda ... etc. from smooth and supple to gnarly, plus everywhere in between. We have f/r discs, but even rim brake 26” tandems with canti or V-brakes will have good tire clearance and will most likely be relatively inexpensive on the used market. With the disc brakes on our bike, I think we could use 650b wheels with 45-48mm tires, but we’d lose the clearance for fenders.
We’ve ridden tandems for 31 seasons and are now sold on wide and supple tires at lower pressure after years of running skinnier 700c and 26” tires at 80-100+ psi. The slight aero/weight penalty has not slowed us down as we’re .4 - .6mph quicker overall (and much more comfy!) with just the tire change.
Last edited by Joint Venture; 12-31-20 at 09:12 AM.