If you need to acquire data from two different operations synchronously, one way to handle that would be to set up on task to drive the timing, and have the second task use external timing.
If we assume that the second task is digital, then the second task would need to use something like "Dev1/ai/SampleClock" as the external timebase. There is no way to start both tasks at the exact same time, so it would also be helpful to use the same terminal as the start trigger for the second task.