As noted, there is a time given on each fast pass that tells you when you can get another FP. After that time, you are free to get another one, for whatever attraction you want.
So, let's say you get a FP for Soarin' at 9:10am. The return window time is for 3pm-4pm. There will be a time listed that you can get another FP though...it might be for, say, 10:30am. Even though you haven't used the Soarin' FP yet, you can still get a second FP for a different attraction. So, you head over to TT at 11...you get a FP for TT return at 2:30-3:30, with another FP available time of 2:00....you head to M:s at 2:05 and get a FP for that...now, you have 3 FPs in hand.