It _can_ be done, I forget the name, but there was nationally funded expedition that used two kayaks tied together just as you subscribe to explore the artic. Also Derek Hutchinson has a picture of such an arrangement in the back of his book. Sorry I don't have a scanner.

Below is a picture of a 21ft Wharram cat, this design has circumnavigated the globe. Take a look at how they are lashed together. You will need to put full wooden bulkheads in where the crossbeams are attached, 3/8 at least. Take a look at the CLC SailRig page to see how the crossbeam are through bolted. I would start off with straight piece I beams like the cat pictured below.

You could also look at outrigger canoes.

The link gets you to the for sale site, so you can look at various boats. If you get on the mailing list they are very helpfull folks.

let me know if this is helpfull

Good luck, and let us know what you come up with.