I think many of us with hub-drives that don't give quite enough power to get up steep hills have thought about this. As Scotty says, you can do it with one battery, two controllers and one throttle, but it's complicated and needs a moderate amount of electrical/electronics knowledge. Most of the one's I've seen had two throttles (one on each side) and two batteries - basically two complete independent systems, which involves a lot of weight and wiring. It would be much easier and cheaper to get a single higher powered motor.