We've done a handful of trips where we booked two rooms. It's been me and my wife (each have APs) in one room and her three young adult daughters in the other (at least one of them has been over 18 each trip, none have APs)
Most trips, we were never asked for our APs (to be fair, I usually have my AP on my lanyard while checking in, so maybe they saw it and didn't ask... or maybe they didn't care).
Once or twice, they've asked to see an AP, but not both (I check in while the others sit in the lobby, so they wouldn't have seen 2 APs)
And one time, the desk agent said that, not only did we need 1 AP for each room, that we had to have an AP holder in each of the rooms at check in.
She said she'd have to put me in one room and my wife in the other. Then put one of the daughters in my room, and the other 2 with my wife. She said we could move people around later, which we did.
I think the answer to the question is - it depends on the desk agent you get. As indicated above, the Ts&Cs say "one room per Pass credential". That's not always followed, but sometimes it is.
As long as you book at the AP rate, if the desk agent follows the policy, you could add the second AP later in the trip and still get the rate - just show both APs at the desk prior to checkout.