So... I was cleaning my shower Saturday and cleaned a little too hard... I pushed a couple tiles right through the (wet, rotten) greenboard it was glued to. I ended up taking that whole section of wall down. The original construction was studs, 1/2" sheetrock, 1/2" greenboard, tile.

I added extra studs (there was only one in the ~30" span of wall), and I'm wondering.... is it okay to use two thicknesses of 1/2" Wonderboard, instead of sheetrock plus Wonderboard, behind the 4" wall tile?

One sheet of Wonderboard would be enough, as opposed to buying some Wonderboard and a sheet of sheetrock.