Many of the attractions at Universal Studios won't open until later in the day, especially in September, so I would suggest starting at IOA. If you're interested in Harry Potter, head straight back to the Wizarding World and get in line for Forbidden Journey right away. After that take some time to explore the Wizarding World. Then you can work around through Jurassic Park, Toon Lagoon and Superhero Island. If there's anything you want to do in Seuss Landing you can hit that up before you head out.
By the time you get to Universal Studios everything should be open and you can work your way around in either direction and make a full lap of the park. If you have some extra time at the end of the day you can easily head back to IOA again if there's something you want to do again.