Current subfloor is 1/2in plywood which will not adequately support any
ceramic tile. But another layer of 3/4in will cause height problems.
Another 1/2in plywood followed by Ditra and tile will just do it.
Now here's something I found at
"Adding the same thickness plywood doubles the stiffness. Gluing it
increases the stiffness 8x! Depending on the tile you might want to
install would depend on whether it is necessary to either just add
another layer, or if it can be glued together. If you decide to glue,
use something like Titebond II, use a squeegee or something similar to
spread it evenly, then use deck screws to put them together. Avoid
screwing the second layer into the joists (but make sure the first
layer is solidly attached before adding the second)."
And in response to the question about not hitting the joists:
"The goal is to make the floor stiffer, but, to also still allow for
independent expansion/contraction of the subfloor materials. If you
attach the second layer to the joists, then it isn't independent. Now,
if you are going to glue them together, some of this is a mute point,
but, you still want to offset the end and side joints, since bridging
them makes the floor stronger - i.e., you don't want the edge joints to
align vertically between the panels."
I'm not in the floor business, so I don't get it.