If anything I would think that the swinging seat, that Softride has, will lessen the force peaks in a traditional down tube and possibly increase the forces in the top tube. I have only worked on a few dozen over the years and saw issues with seat beam pivots and the frame right about the pivot "braze on".

The few tandems I've come across with beams have a curious reaction from the riders. Stokers really likes the suspension, captains liked that their stoker was happy but some disliked the added body movement. Andy