We did this just this past Monday. We already had tickets for Islands of Adventure, we decided at the last moment to add on Universal Studios in order to do a few of the rides there. I was very dubious whether or not this would work. I had not been to Universal Studios in years but remembered it as a "boring" park.
Well, it has certainly changed! I was really glad we added it in. We got there a little after opening, around 9:30 a.m. I wanted to start in Islands because I didn't want to miss anything there, but I was overruled by the others in my party (there were four of us women, aged 38, 36, 23, and 20, there as friends).
At Universal:
1. The Simpsons
2. Men in Black
3. The Mummy
4. Jimmy Neutron
5. E.T.
And we stopped and had a picture with Homer and Marge, and Scooby and Shaggy! These rides were excellent. The longest lines we waited in were for Simpsons and Jimmy Neutron, both about 10 minutes. The others were walk on.
At Islands:
1. Spiderman
2. The Hulk x2
3. Popeye
4. Dudley Do-Right
5. Jurassic Park Pteranodon Flyers (playground was closed

)
6. Jurassic Park River Adventure
7. Dueling Dragons x2 (Fire AND Ice!)
8. Seuss Trolley
9. Cat in the Hat
10. Caro-seuss-el
11. The Hulk
We did close to EVERYTHING at Islands and it closed at 6. We easily could have fit in Dr. Doom's and Storm Force Accelatron if we'd been so inclined, but riding the Hulk several times was deemed more important. We skipped all the shows, which is too bad as I really like some of them, but they really eat up time quickly.
Pteranodon Flyers and the Seuss Trolley were two I had never done before, even though I've been to Islands four times, as they were closed or not available to adults in the past. The line for Pteranodon was easily the longest we stood in for the day, 40 minutes, but it was actually really fun, although only a minute long. I was glad to see what it was all about but wouldn't wait 40 minutes again. The Seuss trolley line was also very long, about 30 minutes, and not really worth it. I really liked the caro-seuss-el, which I usually skip. It was so pretty at night!
I think if you go in knowing what you would like to do and see, and with a pretty good schedule, it's easy to get a great experience out of both parks in one day, without feeling too rushed. We had a GREAT day and thoroughly enjoyed ourselves. We do live in Florida and had all been to both parks before, though, so if you're first timers it might be harder to manage.