Is it permissible to explain the situation of a two-faced person, specifically to my sheikh, or is that considered backbiting (namimah)?

A two-faced person is someone who approaches some people with one thing and others with another. This is blameworthy in Islamic law if their intention is to cause mischief and spread gossip. However, if their purpose is reconciliation, then it is not blameworthy. It is advisable to describe the situation of the first type of person without exaggeration, and to limit oneself to their actions and words.