We have done this many times, even up to 3 resorts. Will not do that again, 2 is the limit now. I also will not do it unless I can have at least 4 nights at each resort, anything less I felt all I was doing was checking in, getting settled and then checking out again. Also the family members in our extended family group that have small children will not do this again. They really missed having a room in the heat of the day to take a break to and let the little ones nap. The families with older kids managed well.
Advantages: new experience at new resort.
If you do as OP has and book a MK resort and a MGM/Epcot resort you get to plan your park touring around your resort. This is really nice.
New places to eat and new pools to enjoy.
Feels like 2 vacations.
Disadvantages:
You love the first resort, get the dream view or location and have to move
On the day of the move you will not have access to you luggage until after 3pm or possibly later, so plan accordingly. One party in our group did not plan and made a PS for 5 and there was a delay in getting their luggage and they had no fresh clothes to change into for dinner.
The day of the move you are basically without a room. You might get lucky and get a room early but then again you might have to wait until late afternoon. Again plan accordingly by planning a day in the park and arrive to the new resort later in the day. This works great for most but things can happen during the day, such as a sick child, where you really wish you had your room back from the previous resort.
Luggage transfer: Let the Bell services of your first resort know the day before you are going to do this. The day of the move call and ask them to come to your room to get your luggage. Someone will have to stay with the luggage until it is picked up. They will tag each piece and give you the receipts for it. Be sure to keep this and make a mental or written note of how many pieces so you can make sure they all arrive. We did have one piece walk off by itself on one of our moves and was not found until several days later.
After they pick it up in your room, go and enjoy your day. Later when you check in the new resort as long as it is after 3pm you can call Bell Services at the new resort and ask for them to deliver it to your new room. Again someone will have to be in the room for delivery. Most times it has been there by 3pm, a couple times it was not. Be sure before the bellman leaves you have all your pieces.