I'd be tempted to say yes, *if* your purchase the 2park/1day express plus tickets.
Or if you plan to ride only a few rides at each park. (i.e. the "thrill" rides like Mummy, MIB, Dragons, Hulk, Fear Fall, JP River Adventure)
However, even with the passes, you will probably have to move pretty fast to get it all done, especially since the parks will probably be closing pretty early in septemeber (6 or 7pm I believe).
If it's at all possible, I really suggest spending two days (1 at each park), the themeing is really very impressive (especially at Islands of Adventure) and it's alot more fun if you can slow down a bit to "take it all in".