Egor, thanks for input.
i chose the simplest, cheapest route. aside from multiple controls, it is also the most accommodating. the engines are mounted side-by-side, never contacting each other. they will be bolted onto individual mounts, then onto a 1/8" metal plate. each engine can be separately adjusted for chain adjustment towards the jackshaft. the 1/8" plate also mounts the jackshaft; this plate is bolted onto a 3rd plate, which is welded to the bike's frame. the 1/8" plate has sliding adjustment, so the jackshaft-to-rear sprocket can be tightened.(of course, there is some chain adjustment at the rear dropout, but it also affects the primary chain on the right side.)
each engine has entirely separate controls. at rest, each clutch lever is pinned, so the bike is freewheeling. one lever is unpinned, then one engine is bumpstarted(or pullstarted at the jackshaft with the rear tire off the ground). with the 2nd engine primed and ready to fire, its clutch lever pin is released. the running engine will bumpstart the 2nd engine. now both engines are warming up, with the rear tire off the ground. when ready. both levers are pinned, and rear tire is placed on the ground. the rider pedals a short way and engages one or both engines.
if one engine falters, or whenever the rider chooses to, either engine's clutch lever can be pinned to remove the engine from the drivetrain. if one engine dies, disengagement will allow the rider to drive home under power.
the builder could also choose an 80cc engine for torque, and a 48cc engine for rpm and better gas mileage. at any given speed, the 80cc engine could be disconnected, and the smaller engine can wind to higher rpm or attain better gas mileage.
with so much low-end torque and more clutch area on tap, one MIGHT be able to pull away from a standing start. however, it would be prudent if the driver pedals a few feet before engaging one or both engines.
centrifugal clutches would definitely be advantageous and convenient, especially at startup and while idling. personally, i just want to finish the project and enjoy the ride.
Myron