Are 1-day, 2-park tickets now commonly avaialable? I'm not sure ...
Regardless, with Priority Access for staying onsite, you can easily do most of the good stuff from both parks, I have done that several times.
In my last visit, with my parents we did all of the following in one day:
* Hulk
* Spiderman
* JPRA
* Dueling Dragons
* Cat in the Hat
then on to USF
* Earthquake
* Jaws
* Kongfrontation
* MIB
* Back-to-the-Future
* Twister
* T23D
* Wild Wild West Stunt Show
* Animal Planet Live
* ET
We also had a leisurely lunch at the NASCAR cafe in there between the two parks.
This was done, at the most, in eight hours, no early entry and we left fairly early (around 5:00 PM ... we were on a 7:55 PM flight home).
This was in late April on a Monday.
If you were to go earlier/stay later, you could easily fit more rides/shows in there.