Hi there!
I know you'll have a great time visiting Universal here in Florida! There are certain "new" rides that do not have the Express Pass activated on them yet, so I would suggest to ride those rides first thing in the morning (possibly even split them into both mornings that you are visiting). Hagrid's and VelociCoaster the the main ones without Express Pass access. The parks do get busy in the morning but then steadily busier throughout the day, but with the Express Pass, you shouldn't have any issues getting to everything in two days. The park hopper is also a great option as it is required to experience everything in the Harry Potter themed lands. I hope you enjoy, Universal is top notch at operations and you should have a very easy and fun two days at the parks! As an insider, I would recommend working your way through Islands of Adventure first, then Universal second.
-Michael, FL Regional Rep