The API function only permits the suspension of one task at a time. If you want to atomically suspend two tasks then place the two calls in a critical section - but note this will only work if not suspending the current task as a critical section does not prevent a context switch.
Likewise, when resuming the tasks you would have to ensure that both tasks have lower priority than the task resuming them, otherwise a context switch would occur. This could be achieved by temporarily raising the priority of the calling task to above the priorities of the tasks being resumed.