I am hanging a double layer of 5/8" type-x drywall with Green Glue, a soundproofing compound, sandwiched in the middle. If I hang the first layer with the face facing outward like normal and then stagger the second layer over it, i'll have about a 5" pocket where the two joined beveled edges of the first layer would recess away from the first layer. My understanding with Green Glue is that it needs to be between two rigid layers, so this 5"x8'-wide pocket would present an area where the sheets are not touching.
My idea is to hang the first layer with the backing facing outward. When I hang the second sheet, the two layers will be back to back with no pockets in the middle.
It may not seem like there could possibly be any issues with this, but my experiences in home improvement have taught me to ask even the most loaded questions since it's better to risk looking like a fool up front than be in a full body cast down the road.
Edit: I am also aware that perhaps a pocket of dead space would probably only server to make the structure slightly more soundproofed. However, I think its best to have both flat sides facing one another to aid in the spreading out of the glue.
Best Answer
Green Glue does stipulate to be used between rigid layers. However, even the beveled edge of the drywall meets this definition.
The edge of the drywall is beveled so that the mud and tape when finished doesn't create a "hump" in the wall at the seam. If you don't mount the drywall with the beveling in the right direction (towards the room) then you will notice all the seams.
Let me preface by saying that I think the green glue installation will be just fine even against the beveled edge of the layer beneath but if you still want to make sure the surfaces are smoothly mated then you should install the inner layer backwards and the front-layer correctly.
That is, the stud-attached layer has the beveled side facing the studs. This leaves the rough edge (without bevel) facing the room. Then the finish layer adhered its rough side to the wall... thus the finished, beveled side faces the room.
On a side-note, are you also using low-density soundbard and insulating the walls between the framing members?