We just got back on Tuesday and I found FP+ would not let me book anything for another park until I had used the passes from my original park that day (didn't matter whether I had 2 or 3 booked). So for example if I had booked 2 fastpasses in MK for 9am - 10am and 10am - 11am then it wouldn't let me book for say Epcot until after those fastpass time slots had passed, so after 11am.
Was a bit frustrating when we were in a park in the morning and wanted to book a different park for the evening but no real biggie in the end. I found most things available every day I tried apart from the big hitters (7DMT, Test Track, etc.).