Safe to put tiles on top of two 3/4" fir board layers & Ditra-Heat?
if you install over wood, you will eventually have cracked tiles and/or grout.  it is not a stable substrate.

If height is an issue, I would take out the tongue and groove flooring, and install the cement board on top on the 3/4" subfloor.  

For floor, i use 1/2" cement board.