I've got a second iteration of my first one in my head...but regretfully, no sketches. It consists of a flip down rear rack and mast support that hinges on the rear cross bar and might even include a locking system on the hinge (locking vertical). Two members attach to the trailer forward of the rear and hinge up to be bolted in place with some speed knobs.

The front rack would be completely welded (as was my last one) and rest on the cross member just in front of the beam of the lower boat. It too would have a brace that goes rearward to bolt to the trailer to give it some fore/aft stability but that brace would be welded to the doublestack rack and be bolted semi-permanently in place.

I've seen some designs that have a box built around the lower boat with a diagonal cross member between the top and the bottom. I like that solution because it gives you complete access to the lower boat...but, the tradeoff is that it makes it forever difficult to get to the trailer boxes.