The Tile Council of North America (TCNA) does LOTS of testing and they've determined that gluing plywood to a plank subfloor isn't the best way to do it. In a conventional installation, they DO call for glue on the top of the joists when putting down plywood subflooring. They also require those planks to be T&G, and well fastened to the joists without movement prior to adding the ply. The spec calls for them to be perpendicular to the joists, like the new plywood, too. If yours are diagonal, their individual spans are longer, and they aren't as strong, and a thicker layer of ply is prudent. You don't need to reinvent the wheel...all of the relevant testing has been done. You can buy a copy of the TCNA handbook, it covers almost anything you need to know on how to do things with tile and have it last.
They also do not recommend glue between sheets of plywood, especially typical, thick, construction adhesive from a caulking gun...it's too hard to get intimate contact of both layers, and in most cases, you'll end up with thin voids and therefore voids between the panels two panels held apart by beads of not quite flattened adhesive (not likely on a thin joist top where the fasteners will apply more force psi). You COULD do it, but if you did, you'd want a full contact with a liquid wood glue, similar to what is done when plywood or glulams are made and LOTS of fasteners, done properly (rather than the giant presses used to make those engineered panels), to clamp it together while the glue sets. It's just too hard to do a good job, and not using it ends up better most of the time.
