Succinct,but hardly informative,do you have specific experience with something along this line ?.With a dual engined bike you have twice the torque and therefore would need less reduction,how much would depend on your particular application.I'll admit it looks a bit on the freaky side.Speed demons@racers with money to spend might go for something like that.Another possibility would be to team up with a front electric drive instead of a IC engine,preferably of course with regenerative capability.Good for hilly terrain, electric motors can be overloaded for short periods (hill climbing) and you could use regenerative braking and direct battery charging when cruising along.Would not be cheap though.