On a September weekday, it will definitely be possible to hit the highlights of both parks in one day, even without express as long as you start early.
For example a couple of Septembers ago we arrived at IOA at opening and rode:
Hulk
Spiderman
Both duelling dragons
Jurassic Park
Then hopped to Universal and rode:
Mummy
MIB
Twister
Earthquake
ET
Shrek
Terminator 3D
Jimmy Neutron
And were finished by about 3:30.
You won't be able to do everything, but that just gives you an excuse to come back again!