I'm assuming you mean the power booster, not the dual resivoir master cylinder. I believe the dual diaphragm boosters hold a higher pressure usually required in 4wheel disc brake applications. I've also heard they work better with engines that have lower engine vacume. I also know part of the advantage is they can get the stopping power of a larger single diaphragm unit with a smaller diameter package. hope that helps.
The advantage is more diaphragm area in square inches that reacts to incoming atmospheric pressure, resulting in higher levels and more linear pedal assist.