I did some searching on the boards because I saw a discussion of this the other day. This is what I found:
1. On your first reservation, say POR you purchase park tickets for the entire trip and the dp for the time you are at POR; on your second reservation, I'll guess Poly, you purchase a one day base ticket with the dp. You save the one day base tickets for your next trip and upgrade them then to cover your new trip. The one day tickets will not expire until they are used (activated).
2. One other option worth trying - there was a post about someone who emailed Disney reservations with their predicament and phone number. A few days later a cm called this person and set the reservation up so that they weren't out any extra money. The cm explained that most of the people taking reservation don't know that it is possible.
If I were you and the first option doesn't throw your budget completely out of whack then I would make the reservation, make a payment; but don't pay the trip off now. Send the email and if you get a good response then the cm can still help you out. I say, "Don't pay it off" because on our first trip my husband thought that he was getting a huge raise and booked a stay at the Poly paying rack rates and airfare. Then we found out that there was a huge misunderstanding and it was a little raise. Since we had paid off the trip, Disney refused to budge on the price or refund the money even after we found a discount code because my husband had paid for the trip in full - at which point the reservations are locked in. The cm said that until it is paid in full you can make any changes you want. I explained why my husband was so upset to his boss (his dad) and we received a bonus to pay for the trip.
I hope that everything works out for you too and that you have as wonderful a stay as we did after getting that bonus.
Regina
) about my difficulties deciding where to stay- BC or Poly or POR. Thanks to recent posts- hello- 
we can split it! 
Boy will that save some $$$$
