In
Disneyland, we got fastpasses at DL, then went to DCA and got some there, then went back to DL to either ride the fastpass ride or get another fastpass. The 2 CA parks don't have their fastpasses connected so I imagine that the ones in FL don't either. Also, you can use a fastpass anytime after the start time on it, not just in that one hour window, so if you get a TT fastpass, then go eat or go to another park, you can come back and use your TT fastpass later that day. We asked several CMs about this as as our fastpass for RnRC was during our dinner PS. The CM there told us we could use it anytime after the beginning of the hour listed, just not before. We also asked at ToT, TT, SM, Splash, BTMRR, Peter Pan, and Pooh. All of these CMs agreed that they can be used at any time. Also, over NYE, a CM at TT told us that since the ride broke, we could save our fastpasses for another day or even another trip. Well we did and used them over President's Week with no problems.
We also usually give ours away if we don't plan to use them, or if someone decides not to ride a certain attraction. I do have a question about that though. It seems as though everyone on this thread is in favor of giving away fastpasses, but they do say nontransferable on the back of them. So to me, this is the same as giving away my 4 day park hopper pass that has a day left on it if I don't plan to use it. Once you use a fastpass, they take it from you so they can't mean that it's non-transferable after being used by the original holder (which I believe is the argument used with not being able to give away a park hopper). Both say nontransferable, so what's the difference? To me, both are acts of kindness that are acceptable.