Are you thinking of doubling the battery packs or just moving the battery pack to the rack?
It just the Radrover is already so tail heavy with the hub motor and I have a rear rack with Topeak bag with foldout panniers on top of that. I occasionally carry my wife spare battery from her Radrover in my rack bag if I'm taking a extra long rides alone. Switching out battery pack only takes a few minutes and I usually need the break off the saddle anyways.