Unfortunately, the requests must be the same through all sites. That is, if you request (or reserve) four nights with Marriott, your comparison must be four nights. If some of those four nights are lower than the Marriott rate, they will approved them on those nights.

If the only way to get it is to make two reservations, that's a bummer but that's how it has to be. Usually you can have the front desk make consecutive stays in the same room.