It happens from time to time, but most of the time it doesn’t. When it does happen, the second group just waits for the first group to finish.
I’ll add this to before people question a tournament dragging on too long. All my tournaments start sharply at 1pm, all the time, every time. Qualifying goes for a guaranteed minimum of 7 rounds. We usually finish qualifying around 4:15-4:30 at the latest. I give everyone 25-30 minutes to regroup, eat, whatever and I start finals at 5pm. All matches are done and all awards handed out before 8pm. This past Saturday with 48 people, our final game in “A” ended at 7:39pm. Awards ceremony was done by 7:55pm.