slogger only problem i conceive with that plan is chain run and handle bar space for 2 clutch handles, at least 1 brake for front and throttle, i mean to do the way you said would mean having second china doll hanging in mid air out the side of the bike better more conceivable would be a setup like this:
2 engines with 2 jackshafts, one master one slave. have front engine rn to jackshaft directly infront of and below current engine with second one in front of it going to that jackshaft with freewheel onto second jackshaft going to backwheel and have first engine going directly to second jackshaft as low as you can mount it on seatpost for smooth chain run from front one, can then alter gearing for two speed set up