Salutation for Two Couples with Same Last Name

Dear Dr. Dave and Dr. Dee,

How would you do a greeting in a letter for two couples with the same last name? For example, Mr. and Mrs. Joe Smith and Mr. and Mrs. Don Smith?

Would you write, Dear Mr. and Mrs. Smith (but using the plural for Mr. and Mrs.)? Or, would I write, Dear Mr. and Mrs. Joe Smith and Mr. and Mrs. Don Smith?

Signed,

Correct greeting?

Dear Correct greeting,

Do the couples live in the same house? If so, then the plural, Messrs. and Mesdames can be used. To be less formal, you could use each first name in the salutation.

If the couples do not live in the same house, then separate correspondence would be appropriate.