Depending on how much you want to see and do, one day may be enough.
If you just want to do the Potter rides once, maybe do the coasters twice (those lines are usually pretty short where the FJ and EFG are pretty long so may not be able to repeat with long lines) and walk thru the lands and a few shops, you will be fine.
If you want to see all the shows and eat at both anchor restaurants as well as ice cream at Florean Fortescues, and explore all the stores and interactive wand spots, one day will not be enough. There are three shows at DA, 2 at Hogsmeade, and a few dozen +/- interactive wand spots between the two parks. The Harry Potter worlds are incredibly fascinating to those who enjoy the Potter stories, and those who don't know them will still be immersed in an area the likes of which they have never seen. I hear the Potter areas rival Cars Land in California but have never been there myself.
After spending 5 full days at the UO parks on our last visit, we still did not do everything we wanted. But we are not commando people, we like to explore and immerse ourselves in each experience, both at DW and UO.
So depending on what you want from your day, you might be able to do it. But just a warning- if you love Potter- you probably won't get your fill.