Check out this thread...
http://www.bikeforums.net/showthread...143&highlight=
The guy to PM is asu_gt. He is doing Parkinson's research using a tandem. I suggest checking out the setup used in the lab scaled back for home use.
Tandems represent even a smaller market than traditional bikes and as such the cost is higher. It is usually hard to go wrong when buying a single bike from a major manufacturer no matter which class of bike you pick, tandems are the same if you buy from a major manufacturer. The problem is the cost is more than 2 times that of a comparable single. There are numerous threads in the tandem forums on cheap tandems. IMHO, cheap tandems are a waste of money for the road.
Now, if the goal is to put one on a stationary trainer for rehab purposes, looking into an off brand model that is less expensive MAY be okay. If I were going to put a tandem on a trainer, I would look at Ebay / Craig's list and try to get old Santana, Co-Motion, Burley, Trek or Cannondale in the proper size. The prices will vary but I suspect $800 to $1000 is ball park and one may do better.
Good luck.