I've done both parks in one day, so it can definitely be done. But, of course, your schedule will be very tight and still there will be many things that will be left out. So, my advice would be to research and find particularly what you're most interested in both parks, and make a list of what you want to see, ride, etc, and then go for it. By the end of the day, if you still have some time left, then you can visit anything you missed or just relax and take everything in. You can definitely get a feel of what both parks are about in just one day if you plan right. Make sure to allow some time for lunch, breaks and pictures with Universal characters.
When I did it, I started at Universal, we rode Jimmy Neutron, Shrek 4D, Twister, Terminator, Men In Black, E.T., and Jaws (not exactly in that order). The Mummy was not opened back then. Then we headed for Islands of Adventure, and we rode Spiderman, Hulk, Dudley Do-Right Ripsaw Falls, Jurassic Park and Dueling Dragons, again, not necessarily in that order.
As you can see, you can pack a lot in a day. We couldn't make Earthquake, Back to the Future (now closed) at Universal, and we also missed Dr. Doom's, Popeye and Bluto's Bilge-Rat Barges, Poseidon's Fury at IOA, as well as any of the shows in both parks, but I made sure to add them all in my next visit. Still, I think we made a great schedule the first time, and in my opinion, we managed to ride the best and most popular rides in just a day.
I don't know about the Express Pass, since the Universal version of the Fast Pass was still free in my last visit. But hopefully someone else can help you in that department. Hope this helps.