Here’s my only experience with this - we were working with a travel agent, and wanted to travel between “x” and “y,” and it turned out there was a deal that only started at x+1 date, so she booked us a package with tickets for x+1 through y, plus a one-night for day x. Of course we had no tickets associated with day x, but somehow an extra day’s worth of tickets tied to the package (which, if we’d activated them on day x+1, we wouldn’t have been able to use).
TA then made a big production out of saying that we didn’t tell her we wanted to use tickets for day x, nobody went to parks on arrival day, that day x+1 should be fine, etc. Made me super angry. After getting transferred away from that agent, her boss said that we should be able to pick up/activate our package tickets 24 hours early, but had no answer on FPP. However, she did something weird where she tied our 5-day tickets to the one-night reservation. It was all highly convoluted and I was very dubious that it would work. Somehow it kinda did, but it did not show up correctly in MDE at any point, and if I called Disney to check on it I got conflicting information. But on FPP booking day, I was able to magically book for the entire trip, and when we arrived we were able to enter MK.
My suggestion for split stays like that would be to book two room-only reservations and just buy the number of tickets you need separately. You might have two FPP days, but no hassles or worries about whether you’ll be able to book FPP for your first day or even enter parks that day. If you have already booked a package for x+1 of your trip, maybe you have a travel agent to sort it out?