It would be great to be able to see all of the nighttime entertainment during your trip, but Disney World does make it more difficult at Hollywood Studios.
With Fantasmic and Star Wars: A Galactic Spectacular usually scheduled close together, they are really requiring you to choose which one you want to see. There are some exceptions when they are scheduled more than an hour apart, but those don't happen too often.
So which one should you pick?
If you're a Star Wars fan and have already seen Fantasmic, definitely check out Star Wars: A Galactic Spectacular.
If you're a first-timer, Fantasmic is a classic that you will probably want to plan on seeing.
If you want to see both, you'll usually need to plan to be there on 2 different nights.
Here's some more info on these that you might find helpful: