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This bike you are doing is terrific!! And just the fact that you went from talk to build is impressive, and more than I have done!
Still, I keep staring at this picture and thinking about this; a front engine with a clutch running a short chain to the 2nd engine, which would have no clutch, rather 2 same size sprockets. Off the second sprocket run a short chain to a jackshaft with a clutch, and then a 3rd short chain to the wheel.
I am admittedly biased towards short chains, but there are other reasons. You could start #2 off #1 (only 1 engine to start), gear changes would be easy at the jackshaft, it would be cool on a dual engine to put dual disk brakes, one on the jackshaft and one on the wheel, and the mechanical addition of the jackshaft would only offset that of the idlers, so i see no net loss.
It might be argued that a disadvantage would be running both motors thru the final clutch, but a suitable clutch is available, so that is just more expensive.
Please understand that I am just thinking here, or trying, and in no way am i trying to diminish your substantial accomplishments!! Damn, I wish you lived in my neighborhood, or me yours!!