Oh, I see you actually have the engines. My bad, I guess if i where going to do that i would make a sleeve that would fit over the back of one of the cranks and onto the front of the other cut as sort as pratically possible. I would lock the sleeve on the the shaft with high tensile bolts going through the center of the shaft, you would obviously need the holes setup so the engines are 180o apart in there respective cycles. Then I would make up some sort of support to hold the two together, maybe a nicely machined item made from ally barstock. You would also need to syncronize the carbs.