We could have last year when we were there, but the parks were extremely slow, almost and errie feeling, but it was nice for the rides. We still however chose to do each park on a separate day, as we had purchased a two day pass. Also keeping in mind it was our third time to the parks so we knew what we wanted to go on and what not to waste our time waiting for.
I think you would enjoy the parks much more if you did one each day. My greatest suggestion is to be there before the gates open.
For Islands, run and I mean run (or certainly a fast walk) to the Hulk. You might just get to ride that twice if your there first thing. We found by being there before the gate opened and being one of the first thru, you're ahead of the lines.
For Universal, again, be there before the gates open ,it's an early morning getting up, ready, breakfast and being there before 9am, I aim for latest 8:15 in the parking lot, as it a little walk to the park, but it's so worth it. I would go to the Mummy first, or actually now that new ride Rip Ride Roller coaster, get on those first, then enjoy the rest of your day.
One year we bought the Express Pass, but didn't for others.
Have fun!