well right off the bat, especially since one is doing well & one is not, I would repot them so that they both have their own pot. If you go that route, clean off the dirt from the roots of the sick tree & look at it for infestation. when you are done studying it, spray off the roots with room temperature water to make sure the roots are clean & clear. Repot with new soil for both.
hopefully someone who has owned/does own Fig trees will come along with some more exacting advice.