There are some factors to consider:
When are you going? If you are going during low season odds are better. If you are going in the middle of summer - it's gonna be tough and feel like a marathon.
Do you have small kids with you? If yes, do you think they can handle a marathon day ( only you'll be able to gauge what your kids can handle ). If no, again odds are better.
If you are looking to do a few specific items at Animal Kingdom ( like the Safari, EE and Dinosaur ) and you aren't worried about missing some of the details then you probably can get AK completed and some of MK.
I wouldn't expect to be able to completely do both parts in 1 day, especially when starting out mid-day.