Solution
The DAQmx and XNET modules within one chassis should already share a timebase, which is in turn synchronized to the timebases of other TSN devices on the network.
You will need to trigger the XNET session appropriately. A good place to start is the "Synchronize C Series CAN with DAQmx Analog Input" shipping example (Help > Find Examples > Hardware Input and Output > CAN > NI-XNET > Synchronization).