Completely do-able with minimal welding!
how about two 80cc chinese engines, side by side on a mounting plate, midframe like the single engine. both engines share the same chain-driven jackshaft, which then drives the rear sprocket. none of the sprockets are free-wheeling. chain adjustment to the engines' sprockets made by slotted engine mounting holes. chain adjustment to rear sprocket via axle adjustment in rear dropout. both engines are individually or simultaneously kick-started,then controlled by their own clutch and throttle levers.
actually this idea is not too far-fetched or involve expensive exotic parts and extensive labor. welding should cost less than $100. since two engines measure 9.25" wide, side-by-side, start with a 12" square steel plate, one jackshaft w/3 sprockets, 2 short lengths of #35 chain, t-fitting on the gas petcock, extra clutch and throttle levers and the rest of the single chinese engine kit.
as an added bonus, gear ratios are easily changed at the jackshaft. the rear sprocket remains untouched.
if one engine fails or ya wanna run just one engine, just disengage the clutch and pin the lever.
what i would need is an old cruiser with lots of room in the midframe to clear the crankset.
and i JUST happened to have two new chinese engine kits in my cabinet.
Myron