My experience based on never having done a B2B...
The price to park on the day you arrive is based on the length of cruise that is departing that day. If you show up on a day the Dream is leaving on a 4 day than you would be charged for 4 days. If you show up on a day the Fantasy is departing for a 7 day you would be charged for 7 days. There are so many cars coming and going in the morning I don't know how they would check to see which cares should have left and which ones should stay.