we do split stays often. we do it so we can, not only experience 2 dif. resorts, but so we can but the dining plan for just a portion of the trip.
we buy the package for the first resort, with the tickets and the dining plan. then buy a room only ressie for the second resort.
now, if you transfer anything to the 2nd room key, be sure to keep the old ones, just in case. we, however, just keep everything on the 1st key, and use the 2nd key for just that... a key (and charging priviledges)
we s t r e t c h out the dining plan this way. we save a table service meal to use when we check into the 2nd resort (example: once our 2nd resort was wilderness lodge, and we ate at Whispering Canyon nthat night) because you have until midnight that night. once port orleans french quarter was the 2nd resort. we took the boat to downtown disney that night for dinner. we also saved a counter service credit each. bought GREAT CS meal at their wonderful food court (jambalaya for hubby, I think, and I had the pot roast. got it "to go". put it in the mini fridge.
the next day, brought it back to the food court and used the microwave!!!also used a couple snack credits for chips, etc. for later in the trip. (do NOT try to save the beignets!!!

did not work well at all!)
but if you want the
DDp for the whole trip, you will have to buy one days tickets on the 2nd package. just buy one only. use the first package for the rest of the tikcets as they're cheaper the more days you buy.
you know what? it's not as confusing as it sounds. have fun!!