If you don't want to pay the $80 bucks for plans, buy Nick's book. It has plans for the double in it, and is much less expensive than the plans. If you're too cheap to buy the book, I will share some of it with you. It says form spacing can be adjusted to produce a longer craft. So use your existing forms, increase the spacing between them, and modify the deck to accomodate two cockpits rather than one. You'll have to do a little math to determine the proper spacing of the cockpits, but it shouldn't be that hard. If it gets complicated, use the money you save on the plans to buy a calculator.