Answer:
Mrs.
Edit - if you're talking about addressing a letter to two men - it's 'Messieurs'. For example - Messieurs Smith and Jones. --Snakester1962 (Supervisor)
Re-Edit-- Mssrs. is an acronym for Messieurs and is acceptable in print... but that doesn't mean most people will understand you. "Mr. Smith and Mr. Jones" would be more readily understood.