I run Conti 4000 in 25c on my tandem. Tourers here must really carry some heavy loads! Caution: only get the black on black with the black chili rubber. Other colors are not the black chili. These tires wear well, roll very well, and seem as flat free as necessary.

For loaded touring on the tandem I have Vittoria Rubino Pro Tech (yes, all those words are necessary) in 28c. These tires are a little slower, but have better sidewall protection. Rivendell tires are about as slow and mushy feeling as they come.

Schwalbe Durano are also good durable tires, not as fast as the Contis but they wear well.