Steer backing up with your hands both at the bottom of the steering wheel and make little movements on the wheel. Small movements of the steering tires are ampliified by the tralier (n doubles 2X's as much as a single haywagon or trailer. Always try to stop with everything straight and parallel before backing.
Remember, when backing doubles (or a haywago), turning the steering wheel right causes the rear trailer or haywagon to track right as well (while the lead trailer (or haywagon tongue goes left.
I can back a set around a corner but it takes time and patience.