I have double stacked lightweight 16 ft cats many times with just 2 lengths of 4x2 between the beams. Just put the top boat on upside down. A little padding so the beams do not get scratched. One mast in the foreward mast cradle and the other tied to it but a little below and some foam padding on the rear beam to tie the masts to. Pretty easy and straight forward.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!