Two Stock CB750 engines, One Transmission?
 

Will the Honda CB750 transmission handle double the power without breaking?

The two engines being phased 90 degrees apart, would be putting out one single power pulse every 90 degrees, rather than one double pulse every 180 degrees as they would if they were phased 0 degrees apart, therefor each pulse would be no stronger than a pulse from a single engine, not twice as strong, as in two engines firing in sync.

The pulses would just be coming twice as often.
Would the transmission see this the same stress as with just one engine, or would the transmission need to be beefed up?

Just a remind: the so-called "universal Japanese motorcycles" usually have an unitary contruction of engine and gearbox, so it would require a lot of re-engineering to match 2 engines to a single gearbox.

Yes, the front engine would have it's gears removed, and be connected by a chain from it's crankshaft to the crankshaft of the rear engine, perhaps between cylinders 2 and 3. (and again, 90 degrees out of phase.)

I've already watched videos featuring small motorcycles with car alternators adapted to them, with a pulley attached to their original magnetos.

I'm not sure how that relates to my original question.

The thing is, if alternating the firings from the two engines can maintain the standard pressure on the transmission, two, or more engines can be safely routed through one standard transmission. (The following illustration shows three engines phased 60 degrees apart.) Can someone show me why this won't work? Prefer actual test data.

The only difficulty I forsee is ensuring you don't end up with 2 X torque for some reason which would eat tranny.

Having said that, the pinto drivetrain survived suprisingly long after we shoved a built 302 into it. Snapped the axles on a launch.

The other question is synchronizing power output like multicarbed british cars.

Yes, the torque. Synchronizing the engines would be by chain driving them together, But you may be right about the torque. Since there is only one firing at a time, the torque should be ok at the input, but with the ability to put the torque of two engines to the rear wheel, might break the output gear. I know I could use a compact car transmission, but I would lose the ability to shift and clutch with one pedal. I would have to use one of my hands to shift.

I wager the second crankshaft is a weak spot, vulnerable to breakage.

I don't know why that would be.