In a statement released on November 10th, 2015, the Wizards Play Network announced that Premier Events - Preliminary Pro Tour Qualifiers and Grand Prix Trials - are no longer eligible to be combined with third party events to host a single tournament with a combined prize pool.

From the statement:

This policy was implemented because a variety of third-party Magic events sponsored by different organizations may be offered at any given time, leading to confusion over which prizes and features belong to which programs. Players might expect a certain set of prizes before attending an event, and find the tournament doesn't feature everything they were expecting.

This policy will affect all events that have yet to be scheduled. Premier Events can be held at the same time and in the same location as a third party tournament, but they must remain distinct events.