Building a rack on the front that could carry a set of cat-trax to use as cross bars has been on my list for a while now. I think that 2 separate stands in the back could support another set of trax for the 2nd boat. I think that it would work well enough, if you were dedicated double stacking most of your trips, then you would probably want an option that didn't lock in the lower boat.
I would be worried about the strength of the catrax for the front part since the 80% of the boat weight is carried there and it sees considerably higher cycle loading while on the trailer. But, that is a really interesting idea for the rear cradle and one I might steal. My trailer is already configured with uprights to store my cat-trax so if I'm double stacking, I only need to transport one extra set. Interesting thought. It might lead to needing extra height to clear the wheels...or maybe not.
