I COULD spend 2 days at Epcot because that's how long it takes to walk around but I WOULD spend 2 days at Animal Kingdom Park or Hollywood Studios easily.
I already posted about Animal Kingdom Park. I think the park is under-rated. The animal exhibits are great. If there early enough, you can watch a veterinary procedure at Rafiki's Planet Watch/Conservation Station. Kilmanjaor Safari is great and worth 2 rides in 1 day. Every ride is a different experience. Then the tigers, the bats, etc... Bang on the drums, kid around with the performers in Africa, lots of characters not seen at other parks, fun rides, etc....
Hollywood Studios... I just have trouble fitting everything in on 1 day because all the shows are on a schedule. To do the Jedi Training, one has to arrive very early and enter well before rope drop to sign up... ditto for Toy Story Mania. Beauty and the Beast is a great stage show.. Indiana Jones, Lights-Cameras-Action, Mud Sweat and Tears, Fantasmic, etc... Just try to fit it all in in 1 day. And don't rush too fast and miss the street performers who are all worth a good laugh and tend to be out more just before the parade starts. Don't miss Fantasmic! I can't believe we skipped that our 1st trip!