You can see allot of the parks in 1 day but things will be rushed. I wouldn't buy an express pass until you are in the park and see how the lines are going. The big difference between Disney and Universal is that not everyone gets the Express Passes and you don't have to come back later in the day to use them. If your there, and want to ride, you just walk on up. Of course the ones you buy only get you 1 use per ride.
I don't know your plans but have you thought of staying onsite the night before or of your UO day? That way the FOTL is included with your rate and you can walk on to everything except the flyers in IOA as many times as you want and then you have a better chance of seeing more of both parks in 1 day.
Hope you have a great time and don't get as hooked as most of are here on the boards