If I am understanding you correctly, you cannot PRE-SCHEDULE fastpasses in more than one park. So you cannot schedule 1 fp in mk, and two in ak later that day UNTIL the first one is used/expires, or (possibly) is cancelled. I say “possibly” because I didn’t test that theory even though the dis help desk told me on two diff occasions that you can cancel existing fastpasses, and schedule them in a different park before you are at that park.
The scenario I gave them was this:
If I go to AK with a fastpass for Safari, Everest, and Primevil, and I use the safari fp, but I find that the lines for Everest and primevil are very light, can I CANCEL those two fastpasses, and immediately schedule two fastpasses for later in the day at a different park while I was still in ak? The answer was yes.