One continuous reservation cannot be divided into two separate reservations . That is true whether trying to do it online or calling MS. To make the reservation into two using points in the intial reservation, you must first modify your reservation to eliminate the days you want to put in the second reservation and then make the second reservation.
To do that can often be impossible because that new reservation is subject to availability at the time the change is made, and, e.g., getting BCV now for early Dec is likely impossible. Moreover, even if you see availability for the other resort before starting the process of making the change, someone else could fill the room you want during the short period of time you are modifying your lead reservation to drop nights, and you could end up losing all those days you take out of your main reservation and won’t be able to get them back.
If you ever intend to attempt to change part of a reservation to a different resort at 7 months out, the best nove is to make two separate home resort reservations at 11 months out, one for the time you want to keep at the home resort and one you will try to switch at 7 months out. That avoids losing the reservation — at 7 months out you will be able to make the change or, if not available, keep what you already have and have MS combine the two reservations.