It is certainly difficult to suggest a title that is appropriate to another person's essay without knowing anything about it; however, since Joseph Conrad's story is strongly directed toward his theme of self-discovery and finding meaning in life through an exploration of one's darker side. So, if the thesis of the essay deals with this theme of self-knowledge, a title can reflect this theme. For instance, "The Secret Sharer":Wisdom out of Darkness or "The Secret Sharer": the Dark Guide to Self-Discovery.
With an approach to the doubling motif, then the title can reflect a thesis that is concerned with whether the narrator has assimilated in himself too much of Leggatt, his darker side. For, at the end of the story, he risks the lives of his crew in order to permit Leggatt to jump out of the ship for Koh-ring. A title for this perspective might be "The Secret Sharer" of the Dark Side.