650B is another option. One of our tandems came with 26" wheels and MTB tires on it and we put road tires on it, but my wife (who captains it with one of the kids) doesn't like the reduced ground clearance. I did some measurements and concluded that a 650B rim would work with the brakes in their current positions (with some reduction in leverage). I ultimately abandoned the idea because it's an inexpensive tandem and there aren't many appropriately inexpensive 650B wheel options out there. I think we're going to just make the bike work for another year and spend some real money on a new tandem next year.