A "split stay" is really a misnomer. It's just two separate, back to back reservations. You couldn't, for example, stay 4 days at Poly and get 3 nights free at Contemporary (or any other combination of resorts). You could only get the free nights at the same resort at which you stayed 7 days. You also would have to buy at least a one-day base ticket for each stay.
Some people did a two week vacation, with 7 days at one and 7 at the other; then it made sense because each leg got 3 nights free.