To clarify, the cDAQ-9172 has a maximum limit of 16 hardware-timed channels of analog output. This channel limit does not exist for software-timed AO.
The cDAQ-9178 has a limit of 16 AO channels only if onboard regeneration is used. If you are not using onboard regen, hardware- and software-timed AO tasks can run up to 128 channels.
So, you can certainly add a second 9264 module to your setup.