+1 on running out the tires that come w/ your tandem, first.

Like zonatandem we're on the lightweight side, but reversegear made an interesting comment about larger tire, lower pressure and rolling resistance. We roll 27" Weinmann concaves (don't ask...) and tire selection is, well, not as broad as 700c. That said, though, we run 1 1/8 (~28mm) on the front and 1 1/4 (~32mm) on the back, not because I like mismatched tires but because the Conti Gatorskins don't come in a 1 1/8, and mounting a Conti 1 1/4 on the front has the bottom of the crown scraping the top of the tire, essentially like a built-in tire saver. I have noticed that the tandem corners well on technical descents and rolls easily, so I'm a bit curious how the rolling resistance bit plays out. 'Sorry, forgot to mention that we run our tires at 100-105, or so. We're pretty light.