As stated by bikerkim, I don't think frame stiffness is going to be a big issue in this situation.

I can tip the scales at 385 lbs. Well if I include the stoker, our bike, pedals, cages, computers, bag, etc.. Oh, I forgot we would have to include the doberman as well (he has to stay home because he doesn't have a seat, or thumbs).

Even as a light weight team, I can speak about the stiffness of a carbon frame. We put our tandems on the trainer in the winter. With a lateral frame aluminium Trek (which I feel is very stiff) I can see a visible deflection of the bottom bracket in that situation. However, with our daVinci (Calfee) I see virtually no deflection on a frame that does not have a lateral tube.