The "straight through" type of drive on the Herse tandem was a very popular choice in France and was, indeed still is, widely used by builders such Follis, Routens, Cattin. As has already been remarked, the system can give a far better chain line.
I have two curved seat-tube tandems a Harry Rensch, and an early 50s Claud Butler racing tandem, but neither of them has a double chainset. However I have a third curved seat tube tandem, a Higgins Ultralite that does have a double chainset working on the straight-through/direct drive principle. In this last case the system has been adopted because the very pronounced curved seat tube is not tubular but made up of thick steel sheet reinforced longitudinally along the edges of the curved sheet with 12mm dia rods bronze-welded and filed in. This type of structure permits an even shorter wheelbase than most systems while at the same time allows the slight concave curve of the plate to act as a type of mudguard. In this case it would have ben absolutely impossible yo have fitted a rear mech on to that structure. Had the much later Simplex b/o that uses a bottle boss as a securing point been available Higgins might just have been able to use it.
Herse and many other constructeurs were known for their ingenuity, so I feel sure that had he preferred not to have the direct drive he would have been very able to have cobbled up some form of fixing to the curved seat tube.
As for the matter of bounce, I haven't found any real problems on the Higgins, but there again that does have a VERY short wheelbase. Haven't I seen somewhere, on some tandems, perhaps on modern "cheapos" some form of frame- mounted jockey wheels to take up the slack chain?
Just an aside..if any of the LIst members intend to be at the Alpe d'Huez stage of this year's T-d-F they could spend a very pleasant couple of hours visiting the huge showrooms of Jean-Pierre Routens that are situated at Gieres, on the eastern side of Grenoble at the foot of the long drag up the valley to Bourg d'Oisans. Daniel Cattin's workshop can be found about twenty kms due north on the main road to Chambery, at Goncelin. Cattin, who is very little known outside France, is one of that country's new wave of constructeurs, although he has been building for at least thirty years now. Lugless frames are his forte.
Norris Lockley...Settle Uk