glasman,
if you look at how a dual master is designed, there is a piston between the front circuit and the rear circuit. That piston is forced into action by the pressure in the rear most circuit in the master. At the rear of the master there is another piston which is has mechanical linkage to the brake pedal.
When the rear circuit fails, the rear piston bottoms on the second piston and then you have pressure in the front system. If the front system fails, the front piston bottoms toward the front and then you have pressure in the rear system.
This explanation is given in Petersen's "Basic Chassis, Susupension & Brakes".
