I'm doing a bathroom with ceramic tile. The subfloor is 1/2" plywood, with joists 16" on center. Can I glue down 1/4" plywood to make it 3/4"? I will use 1/2" cement board with mortar after that. Or can I just lay the 1/2" cement board on the 1/2" plywood?
Yes, if you bond the layers of plywood together as a single structure using construction adhesive. Multiple thin layers that aren't bonded don't offer the same level of rigidity as a single, bonded layer. I'd use a v-notch trowel or a 1/8" bead every 3" (or closer) perpendicular to the joists.
If you were me, though, you'd drop in 3/4" plywood over your 1/2" and call it a day. It's more rigid than the cementboard, will perform perfectly well as a mortar substrate, and saves you a bunch of fitment and fastening of a third layer.