I am contemplating tiling my interior entryway floor. It will be required to buildup the existing entryway floor to match the current living room floor level. The buildup should be approximately 1 and my intent is to layer the buildup as follows:

Two layers of 1/2 plywood (opposing layers), one layer of Hardibacker or cement board.

The current floor is concrete and I understand that I should seal the concrete as a vapor barrier, but is it appropriate use plywood on two layers or should I just layer the whole buildup with Hardibacker or cement board and what could I use to fill any existing void between sheets, mastic? and what am I missing?