Many have this idea they need to do one park a day, but you can hop back and forth.
If you start at IOA first thing in the morning, you can get front seat rides on the roller coasters and get a ride on Pteranondon Flyers before the kid rule goes into effect. After taking a lap around IOA, you can head back to your hotel room to take a rest, then go to USF.
If you want to stay the entire day in one park, that's fine, but my feet usually get tired after standing 4 hours.
The HRH is only a 5 minute walk to USF. You'll be spoiled.