My husband and I have been riding a Specialized mountain bike style tandem for years. We only took it off-road once and that was enough. It's heavy, but we've done a few centuries on it. We just got one of these for the stoker seat and I can't recommend it enough:
http://www.thudbuster.com/
If your roads are as bad as ours, you'll want it! The shocks from thuds and bumps seem to travel through the frame and right to the stokers seat post.
"Bicycling is a big part of the future. It has to be. There's something wrong with a society that drives a car to workout in a gym." -- Bill Nye