I would address it to Mr and Mrs David Keys. For the salutation I would use Dear Mr and Mrs Keys.
Dear Mrs. Obama,
Dear Mrs Ambassador..
Use the name of the person who wrote the letter, and write a formal letter. Use the salutation "Dear Mr. ____" or "Dear Mrs. _____" as a salutation.
Dear Madam (but sign it Yours Faithfully) or Dear Miss..... (sign Yours sincerely) or Dear Mrs..... (sign Yours sincerely)
I BELIEVE THE CORRECT FORM WOULD BE: DEAR DR. AND MRS .....
Dear Dr. & Mrs. Smith
On many job applications, they use the word "salutation" to ask whether you want to be referred to as "Mr," "Mrs," "Ms," "Dr," "Prof," etc.
The proper salutation for a man who is a doctor is Dr. David Jones or you can say Mr. David Jones, M.D. either way is ok. If the Doctor is married: Dr. David Jones & Mrs. Jones.
Mrs, Mrs & Mrs
Dear Judge Smith: See http://www.factmonster.com/ipka/A0771340.html for more info.
Assuming you mean a woman with the title "Lady". On the envelope put "Lady Jane Doe" before the location address. For the salutation in the letter use "Dear Madam" or "Dear Lady Jane" If you are talking about a woman who holds a position of power (e.g President) Use "President Jane Doe" on the envelope. "Dear Madam President" in the salutation If you mean any woman, use Miss or Mrs. before her name on the envelope if you know her marital status, Ms. if don't (or if this is her preference). The salutation should be "Dear Miss/Mrs./Ms. Smith" if the letter is formal, "Dear Jane" if a friendly letter to someone with whom you are on a first name basis.
"Dr and Mrs Smith" is the correct salutation. (With Smith replaced with the correct surname)