2G16_WS90(600)_GFB90_AIR25_GFB90_WS90(600)_2G16 G32_WS90(600)_GFB90_AIR25_GFB90_WS90(600)_G32
Double stud wall with two layers of 16mm drywall (5/8") per side Double stud wall with one layer of 32mm drywall per side
I created these two graphs using insul48SA.exe Sound Insulation Prediction Demo Version software by Marhsall Day Acoustics, although I'm using IRC761's naming conventions for parts.
Basically I was curious to see the effect of using a single thicker layer of gypsum vs multiple layers of thiner gypsum of equivilent mass.
This is predictive software, and not a real world measurement
It may or may not reflect the physics involved in gluing two layers of 16mm (5/8") gypsum together in a laminated way with contact cement. I don't know.

2G16 = two layers of 5/8" (16mm) gypsum.
G32 = one layers of 32mm gypsum.
WS90(600) = Wood Studs (2x4s) that are 90mm thick (3.5") on 600mm (24") centers
GFB90 = glass fiber batts (fluffy pink fiberglass) 90mm thick (3.5") - this might not be what the predictive software uses.
AIR25 = 25mm of air space between the two stud walls

Since the above charts are done with predictive software, I thought I'd show an example of how accurate the predictions are, or are not. IRC761 is a series of laboratory tests done with real walls and are generally considered to be pretty good. Below is a comparison between two IRC walls results and the above insul predicted wall.