Used to own one of these. They are one of the most entertaining things to ride. It is very heavy though, and nowhere near as efficient as a normal tandem, so long rides are challenging. As I recall, my riding partner was usually laughing most of the time! (no matter who it was). We took it to a picnic with about 50+ people, most of whom took it for a ride. The most interesting pair was a 250 lb paired up with a 110lb. It leaned a little, but once underway the centrifigal (sp?) force of the wheels keeps it upright. If you get a chance to ride one of these, don't pass it up.
BTW: Thanks for the links! I plan to contact some folks to see if we can get one of these again!