We love Universal trips! For background, we've typically bought 2-park Seasonal Annual Passes (*note that there are blackout dates for those around Christmas, Easter, and the month of July) as it was cheaper for a 4 or 5-day visit than multi-day passes would be. We've usually stayed at the Premium onsite hotels (although we have spent nights here or there at Cabana Bay and loved it!) so we could have unlimited express pass access -- in most cases, utilizing a passholder discount of up to 30% on the room rate. (You have to stalk the website ahead of time to find the annual passholder ("APH") hotel rates, which are released unannounced a few weeks/months ahead of time).
We also like to visit from Monday-Friday, which means lower crowds and lower hotel nightly rates, and no need to rush around. If you have unlimited Express Pass, you can easily do each park in one day, and spend a third revisiting what you liked best or catching any of the live shows you might have missed. Without unlimited Express pass, you'll need a minimum of 1.5 days per park if you want to see everything, and if you're onsite and don't have express pass, you'll want to take advantage of the early entry hour that is available to onsite guests for one of the parks (it changes every few months). If you do have Express Pass, that's not really necessary, and you'll be able to tour at your leisure. Do stay abreast of what attractions don't offer Express Pass (currently, Hagrid's and Velocicoaster, plus the Pteradon Flyers kiddie ride), so you can be sure to visit them first thing or last thing in the day.
DAY 1 (Universal Studios): Arrive in Orlando and check in at your hotel. Walk (or take the ferry, or the bus, depending on your hotel) to Universal Studios and tour it (clockwise, doing all of the "rides" time allows, plus the Bourne Stuntacular show), perhaps with a dinner reservation at CityWalk. On your way back to the hotel, stop at Voodoo Doughnut and buy tomorrow morning's breakfast. (*Note: We like to start with Universal Studios because it's the less exciting park for us: by starting there, we allow our anticipation to build.)
DAY 2 (Islands of Adventure): Tour Islands of Adventure (all attractions, clockwise), perhaps with a 12pm lunch reservation at Mythos. After a nap break and CS dinner back at the hotel, return and revisit favorites like Hagrid's and the Velocicoaster, in the last 90 minutes before park closing. (*Note: The Harry Potter areas of the parks are particularly, ahem, magical at night, and the Hogwarts Castle projection show is worth catching once.)
DAY 3 (Batting Clean-Up OR Epic Universe): Cover anything you haven't already (especially at Universal Studios, if you arrived late on Day 1 and/or skipped things like the Toon Lagoon water rides), and revisit favorite attractions. If you haven't taken the Hogwart's Express between the parks yet, do so. If Epic Universe has opened, by all means do that, and then do your "revisitations" the following day).
Day 4 (Last Looks): Revisit favorite attractions in both parks, and shop for souvenirs. Play mini-golf or try out the new CityWalk escape rooms, if and as desired. In the evening, take in the Mardi Gras parade or whatever evening entertainment is on offer (or if you're a passholder who pre-registered, or another other lucky guest, RIDE ON A FLOAT AND TOSS BEADS TO THE MASSES, as my daughter and I got to do. It was amazing!)
Day 5: Pack up and head home. Depending on travel schedule, visit parks again if and as desired.
