We just got back from Disney World. The way the rules are on fast pass are as follows:
You must use your current FastPass ticket, or wait 2 hours before getting another FastPass ticket for a different location.
In actual use (while we were there) we noticed that sometimes the time they put on the tickets (the time at the bottom where it says you can get your next fastpass after such and such time) was 2hrs even if the fastpass would be used by then, sometimes it was the time the fastpass we'd just gotten STARTED, and sometimes it was the time the fastpass we'd just gotten had for it's ending time. (example- if time was 1-2pm... sometimes it would say at 1pm we could get another one, sometimes it would say 2pm- regardless of how long in the future the time for that fastpass was).
It was all very confusing. So we just decided to think in our minds that we couldn't get another one for 2hrs when getting the fastpass and then waiting for the ticket to come out and tell us what time we actually could get another one.
I think they should change it to like how Universal Studios does their universal express tickets actually. There you can choose your time. You put the card in and you are given 2 rides to choose from and once you choose which one- then you get to choose between TWO time frames for your fast pass. Sometimes that can come in very handy. Because you can choose the time that flows better with your plans (ie: lunch soon, enough time to ride another ride or something, etc). The only thing I didn't like about Universal Express tickets though- is that they didn't put the time on the tickets letting you know when you could get your next express ticket! LOL