In Sept...both Universal parks in one day?
 

Going to Orlando the week of September 17th. Can I do both Universal Parks in one day or is it really a two day trip?

In my opinion, it's a two day trip. Unless you have boundless energy or are staying at a Universal hotel on site and can go back to the room to recharge.

Or maybe if you've seen the Universal Studios portion on a previous trip and can devote all day to Islands of Adventure, one day might suffice.

Island of Adventure is mostly rides. If my kid didn't want to go on the Hulk and some of the other rides a half a dozen times, we would have gotten out of there a lot sooner.

The older, studio area, is more shows, attractions (parades, outdoor performances, etc) and some special effect type rides (virtual reality type things, etc).

1 day would be tiring, but if there aren't long lines (which we experienced), and you don't need to do absolutely everything, you can cover a lot of territory in a day.

Islands of Adventure has FANTASTIC themeing. If you rush through you will miss all the little touches that make it so special.

Don't miss the Spiderman ride, whatever you do!

It depends on how many rides you want to ride and/or shows you want to see. Whether you do this in one day or two, stay at least one night at a Universal hotel. That will give you "front of the line" passes - which makes both parks much more enjoyable.