A simpler solution might be to make the trailer a 'range pack'. Line the sides with secured batteries, leaving plenty of room for cargo in the middle, perhaps with covers/dividers between cargo and batteries. You would match the secondary pack to the primary(36V for a 36V bike etc)and then add a plug for instant Parallel-voila! almost twice the range.
Admittedly, this wouldn't give you any more climbing ability, but at least your power supply wouldn't drain so quickly...also, i'd be leery of steering with an extra motor loosely linchpinned behind you!
If my extreme poverty and my studies ever let me get an EV bike of my own off the ground, i'll be using a Battery Trailer myself-i'm rural, and the nearest grocery store is a 20 mile round trip...