Basically that was my concern. I was thinking a rubber belt might be better with all the pushing and pulling going on.
I would think that using a v-belt would require some good tensioners........
If I were doing it I'd stay away from belt tensioners. If put on the outside of the V belt they are known in the field as 'belt breakers' because V belts, and a lot of other belts, were never ment to be bent backwards. If on the inside it'd be OK but I didn't need them on my muti Briggs engine set-up. If the jackshaft is fixed [no fore and aft adjustment] the rear axle will have to move in a slot and adjusters will be needed. I know you realize that but maybe that is the place for a chain tensioner. Chains don't mind idlers and that might be easier than adjusting the rear axle. I used parts off a Honda 70 so the axle could be adjusted but I probably won't do that again. The engine bolt slots for both engines will have to be sorta long in comparison to the amount needed if chains were being used. I was able to get away with just useing slots, but no tensioners, so maybe you can too.
Have you decided on how to start the bad boy? Pull both ropes at once? That won't work too good unless you got a helper. But I suppose pulling just one rope will get the job done. I've never tried it. I cheated and mounted an electric starter.
Throttle cables can also become a pain but I know you can do that.
As a Briggs commonly blows its exhaust straight back the first engine will be blowing smoke on the second. I mounted some curved pipes to avoid that.
The minibike frame will be getting kinda long to accomodate the two engines and the jackshaft. I decided to use thin wall 1" X 2" rectangular tube as the bottom frame rails thinking that the usual material, 7/8" or 1" OD tube, would be too weak. Didn't do no 'finite element anaylisis' or any other high zoot stuff but I'm thinking you should look at that regidity issue.
Good Luck!