The Unofficial Guide’s 1 day, two park Wizarding World plan has you starting and ending the day at Universal with Islands of Adventure in the middle. However, on the day I am visiting, Islands of Adventure has early entry which I can take advantage of.
Where should I split my Islands of Adventure plan?
I would personally keep that plan. (rope drop USF) There is nothing like walking into Diagon Alley when empty.
Early entry to IOA should let you knock out Hagrid’s and VC with really short waits. Seems worth it for that.
Diagon Alley early in the morning without big crowds is awesome, and that’s how we started a one day two park trip but we didn’t try to come back to USF once we went over to IOA.
What’s more important to you- Getting to see and do everything in WWOHP or doing it “in order of the books”?
I’m a big fan of Early Park Admission at IOA!! If you get to the parks about 40 minutes before park opening you can get to the front of the Hagrid’s crowds. (Or do a couple other rides like Forbidden Journey, Hippogriff and even VelociCoaster!)
USF will not have early entry, so I’ll be walking in with the entire crowd, plus those that jump on the Hogwart’s express right at 9:00 a.m. from IoA will have a head start on us. I think that group is the one that will see Diagon Alley empty!
Book order is not important to me. Seeing all of Harry Potter in one day is.
Well, yes a large portion of the crowd. 
They let you in the park early, and are even able to be in the Gringotts queue before it starts running at opening time. The earliest transfers from HE will be a little behind.
Well, I may have jumped the gun. I got the email from Touring Plans updating the hours of the days I am tracking, and it had only IoA having early admission, but as I check the Universal website, it is showing Universal Studios will also have early admission. So once Touring Plans updates their Lines app, I’ll be able to go back to the Unofficial Guide’s plan of entering Universal Studios first.
Universal finalizes their park hours & operations calendar about 12 - 10 days in advance. Keep that in mind with your planning.
In peak summer dates, like July, both parks typically have Early Park Admission