You can have more than one fastpass- for a couple of different reasons.
1. You can retrieve a new fastpass when (or within 5 minutes of when) your fastpass return window starts. So, if you picked up a fastpass for Expedition Everest with a return time of 10:30- 11:30 you could pick up another fastpass (for Expedition Everest or another ride) starting at 10:30 (it says 5 minutes after the start of the return window, but people have reported being able to get them as soon as the window starts), then ride using the first set of fastpasses. (This is printed on the fastpasses.)
2. You can get another fastpass 2 hours after you got the last set, whether or not the return window has started. Some popular rides (Expedition Everest, Soarin') end up with fastpass return times multiple hours away from the time you get the fastpass (example: you get the fastpass at 11:00 but the return time is 4 p.m.), two hours after you get the fastpass (1:00 p.m., by the example) you can get another fastpass. (This is printed on the fastpasses.)
HTH!