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.
As was mentioned above, as long as you have a ticket for the entire trip, you can book fastpasses.
We are doing a split stay in September, 4 nights at BLT followed by 4 nights at BWV. We planned to do 2 days at Universal in the middle of the trip so we only bought 6 days hoppers. Will be be able to make 6 days worth of fastpasses or only the BLT first 4 days of the trip? I'm assuming I'll be okay but just want to make sure. It seems like most people say tickets for entire trip, which technically we don't have. We only have enough tickets for the days we want to be at the parks.
You'll be able to make 6 days of fastpasses. Fp's are tied to your ticket, not your stay. The stay is only important for when you can make the fp's (60 days prior for onsite). Even though it's a split stay you will be able to make all 6 days at your 60 day window.