Firstly, this is purely conjecture on my part, as I have never tried this..
1) Create as many contact points as you can to avoid any localised high loading (eg, wrap cushions in plastic to go between the hulls)
2) I would suggest you tie the TI to the trailer, and then the AI to the TI, rather than squash them together by trying to use a single set of tie downs. That way, you can apply just enough tension to hod the AI down without crushing the cushioning mentioned in 1) above.
2012 Tandem Island "SIC EM" with Hobie spinnaker!