I believe it is Messrs.
I have done it two ways each I thought acceptable
Mr. Smith and Mr. Jones or Misters Smith and Jones
I would go with Dear Sirs – or softtops first suggestion.
Me too.
Dear Sirs,
A letter of total and complete enlightenment.
Yours faithfully, Richard Henry
you could be more casual about it and just write, you dudes or you guy’s. If you do that make sure to end it with a high five!
There’s always: Gentlemen,
Messrs. is the generally accepted (and proper) way to address two misters.