A good big tire will actually roll easier than a narrow tire, especially on a tandem. Bicycle Quarterly tested a bunch of tires and found the 35mm Pasellas were one of the best for rolling resistance and, you don't need a lot of pressure to roll well with a tire that large.

The 35s are a bit hard to find but the Panaracer T-Serve is readily available in 35mm.

The 37mm Pasellas would be my 1st choice for 700s on a tandem if I knew where to find them. Given some of the lovely limestone roads in your area I would definitely go as fat as possible.

Oops, just found some 37s
http://www.universalcycles.com/shopp...ls.php?id=7608

BTW We run 50mm Schwalbe Big Apples on our 26" tandems, great tire if you have room.