I found this useful info.
There needs to be two layers of plywood, the top layer being spruce or fir (preferrably fir), exterior grade BCX or better. The layers need to be screwed (spec calls for every 6" along the edges, and 8" in the field-- I go every 6" throughout) BUT NOT GLUED. When laying the second layer of plywood in, make sure the joints of the top layer fall at the 1/4 and 3/4 mark from the layer underneath. You don't want the joints in the two layers to be any closer than necessary. Also, when laying them in, leave about a strong 1/16" between the sheets for expansion, and make sure you're laying it in with the grain going across the joists. Make sure, when screwing down the top layer, that you're going no further than the bottom layer of plywood. DO NOT drive the screws into the joists. This completely negates the effect of double layering the floor by transmitting the movement from the joists right to the top layer of plywood. Once it's all screwed down, take any cheap latex caulk you can find, and caulk the joints between the sheets of plywood. The reason for this is those joints are for expansion, as I said before. Now you're going to go over the plywood with thinset. Sorta kinda defeats the purpose of gapping the plywood if you fill that joint up with thinset. That's ALL the caulking is there for-- to fill the joints with something that will remain pliable and at the same time, keep the thinset out. Last prep spec is that just like with cement board, you need to use a fiberglass mesh tape to bridge those joints . Easiest to use is the self sticking tape, and then just go over it with thinset when you set the tile. Last thing that's different and this is paramount-- the thinset. Just about ANY bag of modified thinset will tell you it can be used to go over plywood. DON'T BELIEVE EM!!!!! The ONLY thinset I'll trust is an UNmodified thinset, mixed with a liquid latex additive, full strength, such as Laticrete's 317 thinset mixed with their 333 additive, or Mapei's Kerabond thinset mixed with their Keralastic additive. The reason for this is it'll give you the highest latex content possible in a thinset, which does a couple of things for you. It's the strongest stuff you can find, and it's also the most pliable, so that it'll take the extra expansion and contraction that plywood goes through, as compared to cement board.