Hi, I have decided to use Hardibacker 1/2" for my bathroom. I would like to use it on all the walls including a wall that starts in the shower and travels to the outside of the shower. Behind this wall is the exterior. By code in my area I can not use less than 5/8" wall for walls facing the exterior. Should I double up the length of the wall with 1/2"? If so, should I sandwich adhesive between the 2 boards?