OK, let me see if I can put all this together. The CB750 transmission will not handle twice the power, but the weak link is the primary chain, not the gears, or the dogs?
So, If I could run the primary chains of each engine to a common jack shaft with a gear in the center, and feed a single transmission through a gear/idler/gear setup, what would be the problems?
I still want the two CB750 engines joined at the crankshafts, but I don't need a 9 speed dual tranny.