Hi there,
I am sitting in our condo right now resting my feet from a day at Islands of Adventure, and I will say that I definitely do not think you can do both in 1 day. We had three free day passes to Universal/IOA, so we thought we would do half a day at IOA and half at Universal. (It is myself, my husband, and my three little girls, ages 6, 4, and 1.)
We arrived at 9:30am and when we felt that we had finally done everything at IOA, it was 5pm! The parks close at 6. We walked over to Universal, saw the Shrek 3D movie, and realized that we were all exhausted and called it a day. (We spent 12 hours at Magic Kingdom yesterday, so it was a lot.) We definitely needed another day to do Universal. I had been looking forward to Universal, but I had heard from several people that IOA had a lot for smaller kids. That being said, Islands of Adventure was amazing! I cannot say enough about how much we loved it. It was clean, the atmosphere in each section was amazing, and the rides were GREAT. My older daughter's loved the Jurassic park water coaster ride, and my oldest LOVED the crazy log flume ride. They also loved the Seuss area. My older daughter's only complaint was that she wasn't talk enough to ride The Hulk. But my husband and I both did, and it was phenomenal! And plus, there were no waits on any of the rides. None. We walked onto rides and then rode them again. This was such a treat after yesterday at MK, where the waits were pretty long for some of the rides.
Anyway, I would suggest to do 2 days so you can enjoy the parks and not run around like crazy. There is no way we could have done both in 1 day.
Enjoy!!!
Melissa