It just depends on how much you want to do. If you are primarily there just for HP plus maybe a few other rides, yes, one day.
If you want to do everything, then you really need about 2.5 to 3 days. Depends on how much time you spend on gift shops, eating, taking pictures, etc.
I’d say 1 day each park would allow a nice relaxing time to see about 75% of each park with EP.
Without EP in July, I wouldn’t plan on doing much of anything in one day based on my recent crowded February trip.
I’d look at the attractions and see how many you are actually interested in doing.
Will you do any of the kids rides?
Personally, I think the Trolley Train is worth a ride for the views of the park, and I enjoy the Minion 4-d movie. The new Villain-con attraction might be of interest if you enjoy shooting arcade games. ET is a fun old school dark ride similar to Peter Pan. Those are the ones we enjoy.
Will you ride the water rides?
Personally, we love them, but they get you soaking wet. Like jumped in a pool with your clothes on wet, so we do them when we can go back to room and change.
Are you planning on performing spells?
The spell spots can get long lines, and even when there aren’t lines, it can still take an awhile to make your way through them all.
Will you stop to explore the HP areas in detail, looking for Easter eggs and through shops?
There are tons of great details to take in, but if you’re just there for the rides, you’ll be done much quicker than someone really exploring the area. You can exchange money with the goblins. You can send mail by owl post. You can eat in a HP themed restaurant or try HP themed snacks.
Will you meet any characters?
Will you watch any of the street shows or the parade?
Will you do any sit down meals?
