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.