Is 1/2 inch plywood over 3/4 inch subfloor adequate for 12x24 ceramic tile?
I suppose if the floor won't get wet, it would be perfectly fine to just lay the porcelain tiles directly over the plywood with thinset.

If the 1/2 top layer plywood was glued to the 3/4 plywood, then it's effectively a 1-1/4" plywood.