List their names individually on the envelope and the inside address, each name on a separate line unless they are married and have the same last name and she uses Mrs.:
Mr. Don Jones
Ms. Mary Jones
Mr. & Mrs. Ralph Smith
Dr. & Mrs. Bill Doodad
[Don't put their addresses here if they are different]
Dear Colleagues,
Dear Friends,
Dear Members,
or
Dear Whatever it is that relates them,
If it's only two names, you can repeat them in the Dear line.
If it's two married men, for example (I am from Canada!), or unmarried men, or any two men with the same titles it would be
Mr. Bill Jones
Mr. Don Smith
123 Main Street
Toronto ON M5V 1T2
Dear Bill and Don,
-or- if it's very formal (you are going to sue them)
Dear Mssrs Jones and Smith,
Two brothers:
Mssrs Don and Bill Smith
123 Main St
Toronto ON M5V 1V4
Dear Don and Bill,