To conduct the battle any other way would be freaking insane.
You are asking trump to do something that would be absolute insanity.

ISIS is the weaker of the two forces.
ISIS is attacking Assad more than it is attacking us.
ISIS vs Assad conflict is WIN WIN for us.
Two enemies killing each other.
ISIS attacking Assad ties up his forces making it easier for us to defeat him.
Once Assad is gone, then it will be easy to clean up what ever remains of ISIS.

If we destroy ISIS first then it will make our fight with Assad a level of magnitude harder.