one way to accomplish this would be to drive one set of wheels directly from the gearbox, and drive the other set with a seperate connector chain which is itself driven by those wheels already powered..
this would require adding another sprocket onto each wheel then adding the chain. you may also need to add a tensioner.....as it is a fairly long span for unsupported chain.
also, look through the gallery. teams have been building four wheel drive robots since the start, and none of them have had a ready made two motor gearbox to start with. so you should be several steps ahead!
