Mark At the Boy Scouts Sea Base camp in the Florida Keys I saw the carnage to a couple of aluminum canoes that had linked together in a similar fashion to what you are describing. They used two lengths of heavy wall electrical conduit U-bolted to the thwarts. The forces ripped the thwarts off of the hull. A rigid connection between the two kayaks will impose significant bending, torsional and axial loads where the connecting members join the hulls. When you consider a person and some gear will be in each kayak, there is a lot more load transferred than you will get from a sailing outrigger. If you are planning to go out in rough ocean conditions, your kayaks and connections will have to be able to perform a stunt similar to a sailing catamaran hiked on one outrigger only the loads will be in the outriggers not on the trampoline between them. Unless you are or can enlist a marine architect or structural engineer you are tempting fate. Build the double.

Jeff