Can the rest of your floor take an extra 1/2" - doors, meeting other floors, etc? Two layers of 3/4" plywood is quite a floor. One 3/4", or 5/8" is usually standard. Plywood, if think enough, is okay to lay tile. It's not the material that cracks grout and tile, it's the flexibility and 1 1/2" of plywood is not going to break grout or tile. They used plywood successfully for decades before the new cement boards and backers came out. If you need a little more height, you can also use 1/4, 5/16, underlayment. Again, with your double ply floor, the strength will be just fine. You said you used leveler. How much? IF you wanted to get an extra 1/2", you need 1/2" of leveler. I really wouldn't use that much leveler though - when you're that far off, you need to build it up another way.