If one Tasks starts while another Task is still running and the two Tasks have the same priority, NPrinting 17 will kick off the second Task on what is a black box queueing system.
If the two Tasks both get data from the same sources (QVW/Sense apps), then the second Task will be queued behind the first as they have the same priority.
If the two Tasks get data from different sources, then they will be run in parallel.
Priorities are currently set by the scheduler and are essentially high priority for On Demand/API driven Tasks and low priority for scheduled Tasks.
We plan on allowing admins to manually set priority for Tasks but currently it's not possible.