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). Do not line up joints - overlap them about 4-6" or so. Keep the plywood running across the joists to provide the max stiffness. You should also check the maximum unsupported length of the joists, and their depth and run those numbers through a deflection calculator to ensure you have a stiff enough floor. You need a stiffness factor not only between the joists (provided by additional thickness of the subfloor) but also along the length of the joist which is determined by the length and the depth of the joist. Walls under the floor are good to shorten their effective length. Note, I'm not a pro. There is a deflection calculator over at www.johnbridge.com you can use.
Also, cement board makes a good substrate to tile to, but adds almost nothing to actual floor stiffness. The instructions for putting down cbu (cementaeous backer unit) call for it to be installed with thinset and the appropriate screws or galvanized roofing nails. Make sure the floor meets the stiffness reqs first, and don't expect the cbu to do it for you.