Some good suggestions in this thread, and I'd say that across two days you should be able to do everything. One thing you didn't mention was what days of the week you're going. Universal started offering FL residents a special where they buy a 1-day ticket and they get unlimited admission through Christmas. This has led to very busy weekends, with the typical lower crowds on weekdays. So I'd highly suggest going on weekdays.
A few notes:
-They've closed a few attractions (presumably to save money) because of lower attendance: Twirl & Hurl (Simpsons spinner), Fast & Furious, Fear Factor Live, Storm Force Accelatron (teacups by Hulk), and Poseidon's Fury.
IMO, the only loss on that list is Poseidon's Fury, a unique walk-through attraction that you don't see the likes of nowadays.
-If your group includes big Harry Potter fans, I feel there's only one "correct" way to approach the Potter sections for first-time visitors. This involves treating the Hogsmeade section of IOA as if it were a standalone island accessible only via the Hogwarts Express, as if the rest of IOA didn't exist.
So you start your first day in Studios. After riding a few non-Potter headliners you start your Potter voyage in Diagon Alley, ride Gringott's, visit some of the shops, poke through Knockturn Alley. Then you go out to King's Cross station and get on the Hogwarts Express to Hogsmeade. Visit Hogsmeade, ride Forbidden Journey, Hagrid's, and then ride the Hogwart's Express back to King's Cross to finish out the day in the Studios. Day Two focus on IOA and then finish up anything at Studios you missed.
Of course, if you don't care a ton about Harry Potter you can just visit them however you want.
-Bilge Rat Barges gets everyone who rides soaking wet. Dudley Do-Right is about 50/50 whether you get very wet or only semi-wet. In December I'd avoid both. However when we stay on-site in warmer weather, one of our favorite things to do is to go to the parks first thing, ride Jurassic Park, Dudley Do Right, and Bilge Rat in rapid succession (sometimes riding Bilge Rat multiple times). We then leave the park dripping wet (with some very odd looks from guests still entering) and go back to our on-site hotel for a shower and change of clothes.
-If you enjoy a beer, check out the beers in the two Potter sections, made specially for the parks. Wizard's Brew is only available in Diagon, Hog's Head Ale is only in Hogsmeade, and Dragon Scale is in both places. (I like a Black Hog from the Hog's Head Pub, which is a black & tan made with Guinness and Hog's Head Ale)
There are also a couple Duff beers available near Simpsons.
-If pressed for time, the following are easily skippable: Shrek, Minions, anything in Kid Zone other than ET, Caroseussel, High in the Sky Trolley, One Fish Two Fish, Jurassic Park Visitor's Center.
-Rob
