Looking at the design, it seems to me that the QR on the top section of seat tube would be better placed at the joint instead of at the top. Placing it at the joint would provide better holding of the seat post accross the joint.

Another mode of flexing which hasn't occurred to me before, is torsional movement of the seat post, with the seat tube section rotating wrt the other at the joint. This would compress one strut and stretch the other. Or if one of the QRs is not very tight, it would allow slip, allowing the post to rotate inside the seat tube.