We do split stays nearly every time we go to WDW -- the next time will be in March, between Poly and BWI.
This is how I do it: take ME from the airport to the first resort as always. The last morning of the first resort, call bell services and have them pick up your bags. Tell them you are switching to the second resort and give them a tip. They will put tags on the bags with the name of your second resort and your name.
Go to whatever park you want to that day using regular WDW bus, monorail or boat service. When you are ready to leave, but after 3 or so, take the WDW bus, monorail or boat to your second hotel and check in. When you get into your room call bell services again and tell them your luggage was transferred from the first resort. Tip the people who bring that luggage to your room.
At the end of the stay there, take ME back to the airport. Enjoy!