most cats pass a stability threshold at twice the length to the beam given the height and power in our sailplans...more beam means the boat is more likely to pitchpole. Lifting foils and lower centers of effort in the sail plan can help reduce the pitchpole resistance.

In your case, with the twin rigs, stability should be a little better since the drive of the sailplan is lower to the deck and my first thought would be to target a 2:1 ratio. I don't recall how long the hulls are you are considering but if they're 18' long a 9 foot beam would be as wide as I would go...which gives you plenty of space.

I did watch your portage video using what appeared to be a couple of prindle hulls and that was pretty interesting. I thought it would be harder to move those around like that. That is pretty encouraging and I wonder if you might not find it necessary to fold the boat when navigating some of the tight river/creek part of the course.

One thing I'm interested in is how you're going to design the hinges on the beams for the pivoting/folding operation. The beams, and specifically these hinges, are going to take a lot of abuse and will need to withstand a lot of wracking forces as the boat goes through waves and chop. Even a light chop will constantly try to twist the beams back and forth, fore and aft. I think this would be the trickiest part to get correct and build something that will withstand the stresses it will endure without fatigue.

Have you looked at Yves Parlier's bi-rigged catamaran, Hydraplaneur, that he constructed several years ago? Obviously it's much larger but he did hold a solo speed and distance record with it for a short while. The bi-rig has some weaknesses when one sail shadows the other on particular points of sail. He had rigged up a small headsail on each mast that he could unfurl to help the leeward rig breathe a little better. http://www.youtube.com/watch?v=AY0ssDA70gY

Also don't discount some of the rig stability you can build in (and reduce the structural reinforcement needed in the decks) if you rig some x-bracing between the masts. Hydroplaner has it as does the Hobie tri-foiler.