Glow engines only overheat if they're A: run lean, B: have no airflow, or C: have an airleak. Running two mismatched engines in a vehicle like this won't overheat either of the engines, although the clutches might not be all that appreciative. They'll have to take up the slack of the differing powerbands. If the gearing isn't overdone then the clutches should live fine even with mismatched engines but I still would rather have matching ones.

Ideally I'd lock the cranks together with a chain or something. If one wants to flame out the other one will force it to keep turning, and the vehicle won't get shaken to pieces as quickly.