The system you are looking at with the 2 layers of 1/2" is using a Type “C” drywall sheet rock.
Type “C” gypsum drywall provide a greater fire resistance than Type X that we are normally accustomed too.
Type C panels contains glass fibers, only in a much higher percent by weight. The core of the Type C panels also contains vermiculite, which acts as a shrinkage-compensating additive that expands when exposed to elevated temperatures of a fire.
Hope that helps.