I saw the design for converting a single trailer to hold two sunfish or a sunfish and a laser.
Based on this design Trailer Guide Posts - Small Boats Magazine from Signal Charlie. I was thinking about copying this almost exactly except instead of having solid crossbars, using wide strapping between the posts to cradle the top boat. I would also run a strap from the trailer frame up over the top boat and back down to keep downward pressure.
Thoughts are that the guide posts flex, boats will bounce and it will be hard to secure them snugly. Ask me how I know
The most secure rig we had had an aluminaum cross bar bolted on with ubolts, made the Picker Run from Pensacola to Grand Island NY and back.
Bring duct tape to secure wooden spars to the aluminum spars, as the wood spars flex more...ask me how I know...
You can suspend a third boat below the crossbars.
and the second most secure had 2x4 bunks tied to the guide posts. That rig was for a short jaunt about 30 minutes away, with WINNIE being about 40 pounds lighter..