It is my understanding that both bands will play the same amount of time (my guess is 90 minutes unless there's no opening bands), regardless of who is closing.
Symphony X played first tonight, and they played for about 80 minutes. There were FIVE opening bands, but I only caught one of them. The whole show started at 5, Symphony X started at 8. My guess is that Overkill came on at 10, but I didn't stick around for them.
According to the HOB website for the Chicago show, Symphony X plays from 8-9:15 and Overkill plays from 10:00-11:15. That's 75 minutes per band, exactly what I predicted.
