Ok I did this last year at MGM last year. When u get a fast pass time for example from 9:00-10:00, as soon as the time changes to 9:00, u r elgible to get a new fast pass.
So here's what I did last year. We were one of the earlybirds 2 get into MGM. We rushed over to Rockin Rollercoaster, got a fast pass, fast pass time said 8:30-9:30 the current time was 8:15. We got on the ride and when we got out the time was 8:30 so went to the tower of terror got ANOTHER SET of fastpasses, went BACK to Rockin Rollercoaster with the fastpasses, got off and then got another set of fastpasses for tower of terror, used the current time fastpasses, Then whe we got off it was again time to use the other fastpasses.
It's rare that u can overlap, it mostly depends on the number of people getting fast passes for the particular ride and time of day. It's more likely u can get them to overlap early in the morning. Also want to mention we went on a extra magic hour day so the general public hadn't been admitted in yet.
Hope u were able 2 understand that mess? ha!