As others have stated, you can't do everything at both parks in a single day. However, do you want to see and do every single thing? I think that a better question would be, can you get enough in at both parks in one day to make it worth your time/money? My guess is that the answer is yes. If Harry Potter is your priority, and there are a few other things you want to get in, you can have a ball and feel as though you got a lot in. I agree that having fast passes would make life easier. If you can spend a night at a UO deluxe hotel and get the fast passes that would make your life easier. Something to consider is, how stressful will this be compared to the enjoyment you would potentially get out of it? Sometimes less is more.