I had a split stay like that last year...first 3 nights
DVC (boardwalk villas) 1 night OOP (no DVC availability) and the last 3 nights DVC. 3 reservation numbers...several calls to DVC and Disney and the resort itself.
This was during F & W, so the resort was packed!!!
They explained it to me as if the reservations are on a Tetris board, and they fit you in depending on the number of nights staying.
I was fortunate and did not have to move. Every time I called they told me that they would do the best they could...but could not guarentee anything.
I reminded them again at check-in and they told me that I would not have to move rooms, I did however have to check in and out at the front desk, and was issued new keys.
Hope this helps....
Carrie