Be sure to nail and glue the ply to the OSB to increase the flex resistance. I have also seen 1/2" concrete board glued to the OSB, but only on L-480 floors.
What you have seen does not mean that it meets the TCNA guidelines! While you can glue the second layer of ply, and it can help, doing it incorrectly can make the floor more susceptable to failure. The glue joint needs to be made with a water resistant full-spread glue like Titebond. Failure to get a full contact and coverage will give you grief. Generally, you are better off just providing the proper layers, and installing them per the guidelines.
CBU is never a valid means of stiffening a floor - the floor must be sufficiently stiff prior to it's application. And, nearly all of them require they be bedded in thinset, not to anchor it (the fasteners do that) but to fill in any minor imperfections and provide 100% support. In normal use, the cbu will move with the tile and the areas around the fasteners will be reamed out slightly because the wooden structure and the cement products expand and contract at different rates. CBU will bend over time to conform to the static shape of that that underlies it...it doesn't have all that much resiliency (no decent spring), so should not be included in any structural consideration other than its additional weight.
FWIW, the person involved decided to go with a ceramic look-alike because the trusses were not designed for L/720.