Complex sentence: At the same time Ford mentored his younger brother, collaborating frequently as writers, directors, and actors in each other's projects; but as early as 1917, it was clear that John's star was on the rise.
Split into simpler sentences: At the same time Ford mentored his younger brother, collaborating frequently as writers, directors, and actors in each other's projects. But as early as 1917, it was clear that John's star was on the rise.