We use Schwalbe Kojak 26x2 inflated to about 80 PSI for touring and riding around town. I don't want anything narrower. Riding on rocky "roads" taught me volume is much more important than anything else. Pinch flats really sucks and one can never carry enough patches (and those instant Park patches really really sucks!).

From speed and weight perspective, I don't find them any slower than the 700x35 (Top Touring?) which we used on our previous tandem.