You didn't state if you will have Express Pass. Even w Express pass doing both parks in one day In the busy month of July is unlikely imo. I feel w Express pass you can tour both Diagon and Hogsmeade completely in a day. But seeing the majority of the remainders of both parks is unrealistic.
With that said, to answer your question, I like to follow in Harry's footsteps and begin in Diagon Alley. Complete that, then take the train to Hogsmeade. Complete Hogsmeade, then see where you are w time and hit any highlights that interest you in the other parts of Islands of Adventure. Or, this may be a good time to take train back to Diagon to experience that route, then hit some rides in the Universal side where you will end up again.
It's probably best to decide in advance any must do attractions you would like to try and hit in the 2 parks after wwohp and prioritize them.