I'm going to go against the consensus and say you can almost certainly do both parks in one day, especially if you're not trying to hit every single ride/attraction in both parks. It depends entirely on length of the lines, but September is a lower season than summer and usually has very small lines because so many people are back in school. Sure, weekends will still probably bring out a bunch of people for DA, but if you go on a weekday morning, show up right at opening, and make a beeline for DA, I would be stunned if you have a huge wait for Gringotts. Of course there's some degree of uncertainty because it's the first year/a new attraction/etc., but that doesn't change that very few people can take their kids out of school the first week of September.
Let's put it this way: when we went a few years ago during slow season, it took us less than half a day to do IOA, and that was moving slowly and spending literally an hour and a half taking pictures of details in Hogsmeade. And while there was a bit of wait for FJ, it was nothing like during the busy season - summer had lines of 4-6 hours routinely, I think we waited 30 minutes. So I think it would absolutely be doable if you're going that week.