You can't do Universal Studios and Islands of Adventure in just one day - not adequately anyway. If it were me , I'd do one day at SeaWorld and two for the properties at Universal.
I did do "highlights" of both parks in one day once (we were able to ride Spiderman, Jurassic Park...the small coaster (not one of the big ones)... had our fortunes read and then we left for Universal Studios where we hit many of the main attractions but not all.
It was also exhausting because that is a LOT of walking.