What is the pural of Mr.?

Answer 1

The plural form of the title 'Mr.' is Messrs. When addressing two males by title, the title is plural not the name.
Example: The two Messrs. Smith are cousins.

However, addressing people in this manner is quite formal, it's not normally used in everyday conversation. In general conversation we might say, "The Smiths are cousins."

The abbreviation Messrs. is derived from use of the French title messieurs of the 18th century. Messieurs is the plural of monsieur.