You cannot mix CGX and P96. You can have two or more gfx cards using CGX and you can have two or more gfx cards using P96, but you cannot have one gfx card using CGX and another one using P96.
However, if you are careful, you can install both P96 and CGX and to switch from CGX to P96 or vice versa, move the used monitor driver to devs/dosdrivers and the unused to storage/monitors, then reboot. You might also need to rename some libraries.
But IMHO you don't need both. CGX programs should run fine on P96 (P96 emulates CGX), so you only need to install P96.
Bye,
Thomas