It depends on how much you want to do at each park! With a half day at AK, you can do: (1) the Safari; (2) the two walking trails or Expedition Everest and Kali; (3) Dinosaur or Tough To Be A Bug; (4) one of the three live shows; (5) one of a character M&G, the Boneyard/Triceratop Spin, Rafiki's whatever, the Discovery Island trails, or another skipped attraction of choice; and (6) a counter-service meal. As you can see, you'll have to pick and choose, but it's certainly possible to hit the highlights in half a day (which is what my family typically does, standby only but with a rope drop arrival guaranteeing short lines, and saving our FP+ for our evening park).
As for Epcot, in one half-day you can either do the vast majority of Future World or the majority of World Showcase, or just a little bit of each. The biggest issue (potentially) is the fact that you only get one "Tier 1" FP+, so you can do Soarin' or Test Track, but not both (at least, not without a prohibitively long line). We typically spend a half-day doing most of Future World (Soarin', Living with the Land, The Seas with Nemo, Imagination, Spaceship Earth, Character Spot, Club Cool and Mission:Space -- skipping Innoventions, Ellen, the Lion King movie, Captain EO and Test Track). Another half-day is spent on World Showcase (including all the World Showcase "rides," the American Adventure and one or two of the three films in France/China/Canada), plus Test Track (either with Fastpass or just after rope drop).
