We are doing this same thing in May (split stay between BWI and a rental house with other family.)
When our FP+ window opened up, we were able to book the 5 days of our package. But anything beyond that was greyed out and unavailable for selection. But as we hit the 60 day mark, each day has become available for us to reserve rides.
What is yet to be seen is if it goes by the 10 days of our tickets or if we have the 14 day window that our tickets are active for. I guess we'll find out in about 3 more days time!