If no pedals are involved, then having wider engines shouldn't matter. That solves a LOT of issues. Unsure of the quality of centrifugal clutches out there, tho.
You MIGHT get by with one centrifugal clutch and one standard clutch. That way, you could pull-start the first engine, which would spin the jackshaft and "kick start" (not pedal start) the second one. Or pedal start the second engine, pin the clutch and pullstart the first engine.
Orrrr, you could run either engine solo, or pedal the bike with both engines off.
As long as you have room in the frame, it should be easy enough to install a jackshaft. What I would do is look at the SBP 4-stroke jackshaft/shift kit. SBP has a bolt-on flat mount and built-in j-shaft for the 4-stroker. If your twin-China mounts can bolt onto the SBP mount and align with its built-in jackshaft, then everything will line up with their shift kit. The shaft itself might have to be placed with a longer one. Short chains from engines to jack-shaft would be needed.
What an EXCELLENT project!