It all depends on when you're going. During the summertime, Fantasmic sometimes plays at 9:00 and 10:30. Most times in the year outside summer (and holidays), Fantasmic only plays at 9:00. Illuminations plays at 9:00 nearly every night of the year.
So, if you're going in the summer, it might word out. If you're not, it's not going to be possible.
Both are worth it (Illuminations is my second favorite thing in WDW after Tower of Terror), so I hope you can work in seeing both.
RyMickey