Depends on what you like to ride. If you want to try most everything, you'll need two days for sure. If you just want a brief overview and plan to come back again, you could see both parks, but you wouldn't be able to do most attractions.
You need two days to be able to park hop. I did both parks in two days last summer, but I was only able to ride two rides more than once, and left a little dissapointed, plus I missed three rides. I guess it depends alot on what you ride, and what your feelings are at the time, plus on what time you're going as well.
We had a 3 day pass last summer and although we managed to see just about everything in both parks it just didn't seem enough!
This year we've got two weeks at the HRH including a length of stay pass so we'll probably spend about 5 or 6 days at our leisure around the Universal parks. (FOTL should help too! )
It depends what you like but 1 day would never be enough for us.
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