Success Is Easy
Two Tracking
         To teach a horse to two-track, travel along a fence line and gently but consistently ask the horse to move his hindquarters away from the fence and toward the center of your circle.

         When first teaching this exercise, begin at the walk. No fast action is required of the horse. It is permissible during early training to turn the horse’s forehand into the fence by using an indirect rein as well as some inside leg pressure at the girth. If you are moving to the right with the fence line on your left, for example, push the horse’s hindquarters out with your left leg. At the same time use the right rein, an indirect rein, to block the horse’s forehand movement. This slows the forehand and allows the hindquarters to move out more easily. You can also use some right leg pressure to slow the forehand, actually pushing it slightly into the fence line.

         When two-tracking to the right, for example, the horse will create a set of forehand tracks just off the rail. The horse’s left foreleg and left hind will cross over the horse’s right foreleg and right hind when moving to the right if performing correctly. The horse should create a separate second set of hindquarter tracks toward the inside of the circle--thus the exercise name--two-tracking.

Taken from the online course
Training Performance Horses