Hi there,
I think if you tried to squeeze both of these into one day you could end up enjoying neither of them. I haven't been to Islands of adventure but Universal Studios i have been to. The day we went, we were there before 9am and stayed until after 6pm and
there were still one or two rides/shows we didn't get to.
It's an exhausting experience with lots of queuing in the heat and some physically demanding rides, not to mention the miles of walking around the park during the course of the day. I would suggest do one theme park per day. Anything more ambitious than that I think would be naive and most likely counter-productive.
Just my advice. Others may disagree.