It's been done before, but having both on the rear wheels will have synchronizing problems. Sorta like a tank with twin tracks, or twin-engined airplane.
Maybe one engine on the front fork and another in the rear.
Actually this guy as a youtube video of this dragster gokart they built with two moped/scooter engines. If you go and look up my username(Jax Rhapsody, duh) I should have it in my favorites. If you use the same engines it shouldt be that hard to do.