I've done both in one day on two occasions, neither time using EMH. It will depend on crowd levels when you're there, but with planning you should be able to do the major attractions at both parks in one day. On my trip last month, here's what we were able to do in one day:
Got to AK a little after 9am opening
Everest twice in a row, once standby & once w/ FP
Kali River Rapids
Primeval Whirl
Dinosaur
Got FP for Safari
Lunch
It's Tough to Be a Bug
Safari w/ FP
Left AK around 2pm, got to MGM around 2:30
Got FP for Rockin' Rollercoaster
Great Movie Ride
Star Tours
RnR w/ FP & picked up another FP on the way out (this is my favorite ride)
Backlot Tour
5:50pm ADR @ Hollywood & Vine
Finished dinner around 7pm
RnR again w/ FP
Went in to get a seat for 8:30 Fantasmic @ around 8pm (had the dinner package)
We didn't do a whole lot at MGM, but that's because there wasn't a lot there we were interested in, and we had plans for dinner & Fantasmic. The park also closed fairly early that night (8:30) and there was no EMH. I'm sure if you are able to utilize EMH at one or both parks, and possibly skip Fantasmic, you'll be able to see more of the attractions. It also obviously depends on the ages of your kids and what kinds of rides/shows you like. Have fun!
