As NangSgi indicated, this is not an SSH (Simultaneous Sample and Hold) device, but rather an even better simultaneous sampling device. SSH devices have a single ADC and analog circuitry which "latches" the data on all inputs at the same time. The ADC can then digitize each of the held channels. In a simultaneous sampling device, however, there are as many ADCs as channels.
As for your 9215, it does not support any type of external triggering. To do so, you would need to place it in a CompactDAQ chassis, and use another digital module to accept the start trigger of which you speak. Please let us know if you have any more questions.
Hope this helps,
Ryan Verret Product Marketing Engineer Signal Generators National Instruments