I understand that there are a lot of things that determine how a bike rides, moreso than frame material. That being said, it would seem to me that Ti may not be the best choice of materials for a tandem. Its relatively flexy nature, which gives it the "magic carpet" ride tag, would seem to work against making a tandem stiff enough, particularly for a larger or stronger team.
but again, I'm sure it depends on what your looking for in a bike, and how the particular bike is made.
To those of you with experience, do Ti tandems tend to be more flexy than high end tandems made of other materials?