If I understand you right you have been to Hogsmeade at IOA, but have not seen Diagon Alley at Universal Studios.
Having the park to park ticket allows you to ride the Hogwarts Express between both Potter parks, that's a plus.
There is only one ride in Diagon Alley and that is Escape from Gringotts. Wondering around Diagon Alley and Knockturn Alley is really nice. Use single rider line on Gringotts if needed to get on Gringotts if line is long.
There are a lot of shops in Diagon Alley to look in. Going back to Diagon Alley in the evening when it takes on a different feel is nice to see.
You also have the Ollivanders wand show.
If you are not staying onsite with early entry and unlimited Express pass and you are going during a busy time you will have to hustle to see everything.
Two days would be better than one, but if that is all you have time for you will have to be there at rope drop and stay until park closes probably. That's a long day. We usually stay a week.
If I were you I would go to the Universal web site and look at the maps of the parks again. Get the lay of the land again and make plans according to what rides are most important to you.
If the Potter parks are your main interest then focus on them. Have a meal at Three broomsticks in Hogsmeade
(they have a family feast meal that's very reasonable for a family of 4-6) or Leaky Cauldron in Diagon Alley this makes the Potter experience complete. When you have had your Potter fill then move on to the other rides.
Anymore questions ask away. Someone will answer for you.
Have a great time
