I just finished a tour along the C&O Canal, riding Schwalbe Marathon 26x1.5 tires. These were on my single Co-Motion touring bike, but I'm sure would be fine for a tandem, too. I was pleased with their performance on widely varying terrain on this trip, from almost-singletrack dirt/mud with roots and rocks, to paved roads, to hard-pack dirt and loose gravel. Pretty close to all-round tire IMO if you want a bit of volume. They aren't the lightest tires out there, but with the Green Guard barrier they hold up very well and rarely get flats (I've run 26x1.25 versions for years, including on our 26" Santana Fusion tandem for touring).