You can do FP+ for the whole stay when the window opens for the first room. I just did it for our last split stay. If couse, you have to have a ticket of the appropriate length and the reservations linked to your
MDE account.
You can do the ADRs for both at 180 days, too, but you will have to call MS to book those you want for the second part. Online system makes you wait until the window opens for the second part. MS doesn't have that restriction.