How do you address more than one Mr. in a business letter?

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.