If you haven't already done so, I highly recommend you use the "view all steps on one page" function.
Step 1: Idea Generation Phase
I saw these two hulls lying beside each other and half jokingly thought that they could be lashed together to create a ridiculous sailing craft of some sort.
Basically if you have some ingredients similar to these (unlikely), sit them a nautical distance apart and lay some sticks between them. Now you have the choice of either convincing anyone who is around that this idea could work, or just ignoring the laughter and getting on with it.
Step 2: Let the lashing begin
This fantastic system allowed us to quickly get the crossbeam and deck structure worked out to rough dimensions, while the flexibility and ease of adjustment allowed for fine tuning. The structure we came up with is fairly simple and the photos explain it well enough. Anything with reasonably substantial crossbeams between the hulls and a bit of diagonal bracing would probably work.