To increase the size of a drive shaft, you could use a bushing.
Pure conjecture on my part but for the very few gains of 2 weed whackers, might not be worth it. Will only go ever so slightly faster then the lesser performing engine for double the fuel consumption. The only advantage I can see is a power increase which would be more easily optained by using one larger engine. To sync em, only way I could think of is to use one throtle cable to both and use a small engine tachometer to adjust so the rpm get close with the uni-throttle.
Another option would be to mount one on the front wheel and use a RT & Left thumb throttle. Just a thought. But would keep it simple and only use one engine.
I highly suggest keeping the pedals. Run out of gas or have engine failure and you can bike home or to the gas station. LOL, every one reading this who has had to push a bike home is nodding their head right now.
But they are just some thoughts and I would really enjoy seeing you do this. Would be a one of a kind, for sure.
Post lots of pics!