Pete,

There were plenty of double stacks going on at SF.

There are specialty trailers made for double stacking. You could build your own - but would probably spend more and not achieve results on par with the solutions already out there.

This weekend JC and I double stacked his C2 ontop of my N20. He had some custom fiberglass brackets that fit over my crossbeams that slso mated to trailer cradles. Strap them down and the rig was solid.