It is really hard to suggest the title to an essay without knowing what the focus of the essay is. This said, an essay on Joseph Conrad's The Secret Sharer could be assumed to focus upon many different things. Three themes in the text are initiation/self-definition, doppelgangers (the other), and symbiotic relationships. If focusing upon one of these themes, suggestions for a title would be as follows:
-Can You Keep a Secret? (self-definition)
-The Secret Between Us (doppelganger)
-Sharing Secrets (symbiotic relationships)
Another route would be to play off of Conrad's alternative titles for the text: "The Second Self," "The Secret Self," and "The Other Self."
Other suggestions, based upon ideas in the text, are as follows:
-Navigating the Secret
-Deception in Secrets