How do you address specific men and women together when writing a letter?

When addressing a letter, there are a number of acceptable variations, including but not limited to:

Addressing two men: Gentlemen (without dear); Dear Messrs. Smith and Jones; Dear Mr. Smith and Dr. Jones; Dear Prof. Smith and Mr. Jones.

Addressing two women: Mesdames or Ladies (without dear); Dear Mses. Smith and Jones; Dear Prof. Smith and Ms. Jones; Dear Ms. Smith and Rev. Jones.

Addressing a man and a woman: Dear Sir and Madam; Dear Mr. Smith and Dr. Jones; Dear Prof. Smith and Ms. Jones.

Addressing a group of unknown or mixed gender: Gentlemen and Ladies (without dear).

Source: The Gregg Reference Manual, Sixth Edition