Having a different Start Trigger terminal for each task is normal as each task uses a different timing engine . The NI cDAQ-9189 chassis provide three timing engines for analog input (te0, te1, and ai). The Sample Clock and Sample Clock Timebase, as well as the Start, Reference, and Pause triggers, exist on each timing engine. Therefore, the names of the output terminals for those signals include the associated timing engine.
In each chassis the timing engines share the same clock source and between chassis this source clock is phase-aligned trough TSN Network. Thus by adding a start trigger to the task, the data is synchronized. For more information about synchronization with TSN devices refer to How to Achieve High-Accuracy Measurements With NI-DAQmx-Based TSN Devices .