My family will be coming to Orlando in December. Myself, husband, DD-21, DS-20, DS-19. We have a couple of days left on our Disney passes and will be staying offsite. I thought it would be nice to do a day at Universal. We have never done this before and I was looking around at the website. We are big Harry Potter fans. We are only looking at one day at Universal. Can we do both parks in one day or should we just pick one? TIA!