A dual reservoir MC does not have two independent pistons- - - -they are inline with each other, so there's no increase in volume. A column of fluid between the front and rear pistons actually pushes the forward piston. If there's a leak in one circuit, the pistons come into physical contact with each other, the pedal goes down to less than half its original height, and the remaining undamaged system gives you enough braking to come to a stop- - - - -longer stopping distance, much less efficient, but not a total loss of braking effect. That requires a very much different plumbing setup, and several valves that respond to the loss of pressure on one end to isolate the failed system from the good one. At one time, there was a "dual-diagonal" setup, where each chamber of the master cylinder operated one front and one rear brake. That one was a real nightmare to fix, especially trying to bleed the brakes!
Jerry
