I was told to put 1/4 mahogany over my 3/4 subfloor to tile on.
Do NOT pay attention to that person.
Thanks rusty so I'm assuming that I can screw the half in underlayment down then? On that same pattern
AH.....not sure what you mean, but probably not.
You do not need any more plywood for ceramic/porcelain tiles if the 3/4" subfloor is plywood as apposed to planks. Do NOT tile directly over the second sheet of plywood in your subfloor system. (if you have 2 layers). You need either a tile backer board or a membrane such as Ditra to tile on to.
So, for ceramic/porcelain a 3/4" plywood or OSB subfloor is adequate if in good shape. Then install the backer or membrane and then the tiles.
To make sure your framing meets specs, let us know the type and size of the joists. Their species and grade if at all possible. The spacing of the joists. And the unsupported span of the joists. Also confirm the type and number of layers of the subfloor sheets.