Thanks. That's what I remembered, but I just cannot find anything reference to this from ni.com.
In this case you can actually only run 2 hardware timed analog input tasks with different sample rates because the 9231 is a DSA module. This is because the cDAQ 9189 is a Gen II chassis and Gen II chassis support a maximum of 2 synchronization pulses. This is called out here.
C Series IO R&D