Forcing guests to buy a 2 park pass to experience the attraction is going to leave a lot of unhappy guests in the Universal Orlando resort. After paying 16 dollars for parking and roughly 100 dollars for admission, telling certain guests you can't ride the newest attraction in the park will not go over well.
Also, JK Rowling making guests go through a brick wall to enter Diagon Alley is going to get old very fast. The first time it may be cool, but on repeat visits it will become very tiresome. Not to mention the guaranteed narrow walkways in the new land.
I love Potter, but Disney is way better off buying Star Wars, Marvel, and Pixar then renting Potter and having to deal with that lunatic(Rowling).