There are two competing steering design styles for tandems. The Santana/Burley one is that stoker steer potential is a bad thing and the trail should be less to reduce this. The CoMotion/Calfee idea is to increase the trail so the handling is more "single like" at the expense of greater stoker steer. Also the larger inertia of two riders make any steering geometry feel slower then a single bike's. Have you measured the geometry of a number of tandems yet?

At the shop that i worked at for 7 years that sold all these brands we found that the first tandem was usually the Santana/Burley design. But the second tandem (the upgrade...) was usually the CM/C style. I always thought that the more experienced team had learned to reduce stoker steer by their behavior while getting use to the first tandem.Then when moving onto the next one wanted a more reactively handling tandem.

As to tubing: it's hard to not make a tandem too stiff, especially if touring weight is to be carried. So big tube diameters rule. The walls get pretty thin on the sport bikes to keep the weight down but stiffer is generally better. At one time Santana use to sell there tube sets to privet builders. I would stay away from single bike specs at all costs. Andy.