I had the same questions as you. Look in the liberry. It has some good info. If you do a search, you'll find a long thread from 2000 or 2001 that goes into a lot of detail, and you end up having some questions answered and some not answered.
Dislcaimer: I'm a DIYer
, and can only speak from my limited experience and from what I've read here and there.
First, I don't think you would want ply sandwiched between Ditra and felt. Any moisture that did get into the wood would be trapped.
Screw top ply to subfloor only, do not screw to joists. Another tip was to predrill the top ply before placing screws. This allows the sheet to be pulled tight to the subfloor, leaving no gap from threads catching.
To glue or not to glue, that is the question. Opinions differ, and it's a pretty heated debate; not to mention interesting
. I personally would say that the answer to whether or not to glue is...IT DEPENDS!
It would depend on your subfloor thickness, joist size, second layer of ply thickness, whether or not you're using a membrane (like Ditra), the size of your project, and what materials you're using for tile. You need to way out the need for a stiff floor to the need to allow the floor to have more flex or compensatory movement. Exactly how that is going to be decided is beyond me. I have a 5/8" subfloor and placed a 1/2" ply on top. I decided to glue the two together because I am using slate, and since I will use Ditra underneath I felt it would provide almost all the uncoupling, even if I did not glue, and I needed the floor to be as stiff as possible. I have also placed a beam under the house for added joist support (make sure you check out the deflection calculator). The other thing is my bathroom is small, only 5' by 8'. So movement would have been minimal, and probably would have been of little benefit to keep the plies uncoupled.
Again, I'm not an expert, and this is what I took from what I have read.