Simple Answer: Turning the wheel to the left while backing will cause the rearmost trailer to also move to the left.
In-Depth Answer: When sitting in the tractor looking forward, when you start to back up doubles, while watching the mirrors, the rearmost trailer - not the one immediately hooked up to the tractor - will move in the direction you turn the wheel. A SINGLE trailer will move in the opposite direction, i.e., while you are backing, turning the wheel to the left pushes the rear of the TRACTOR to the left, which pushes the FRONT of the trailer to the left, causing the REAR of the trailer begin to swing toward the right. You have to remember, you are PUSHING the trailer. With doubles, the trailer closest to the tractor in this scenario is moving toward the right, pushing the rearmost trailer to the left. Thus, turning to the left causes the rearmost trailer to also move to the left. BUT, use only small corrections; once you get the trailers out of whack, it's not easy to get them back where you can control them well. And, don't be afraid to pull up a little bit to bring them back into line.