This is how the stays will be treated I imagine: You would get the night/stay credit for the first hotel, a flag for potential account misuse for the second hotel and it wouldn't post automatically. This FAQ has more details.
FAQ: Points & credit for same-night, overlapping and nested stays at different hotels

I guess it sounds in this case like the hotels themselves are closely linked and may be able to help just remove the number from the second reservation: Realistically you should get a second account for the second room, or if there is an overlapping night with 2 rooms, do that as a separate reservation.

The following would be within the rules and maximise points for the Marriott Bonvoy account presuming that you are staying all the time, plus avoid any problems with the status rules being wrongly applied to a room: 1x 3 night stay (Reservation #1), 2 x 1 night stay (Reservation #2), 1 x 2 nights stay (Reservation #3).
This is because of a new rule that says that you can only get points/nights credits for a stay that has the same dates with multiple rooms (new in the past few months). On check-in you could get the reservations linked up. Reservation #2 could be split up into 2 x 1 night as well if you wanted (4 overall reservations).