Consider someone helping two friends resolve a disagreement. The mediator listens fairly, does not invent facts, and encourages reconciliation. That is tactful conduct. By contrast, a person who slanders one friend to another and then reverses the story to secure favor from both is practicing duplicity.
Example 1: “I will not pass on your confidential concern without permission” — protecting trust.
Example 2: “I will tell each side a different false story to benefit myself” — deception.
The expression ذَا وَجْهَيْنِ criticizes two-faced character, not polite behavior. Truth can be maintained while speaking gently and wisely.