Correct way to address an envelope if only the wife is a doctor?

"To Mr. and Dr. Smith."

Or just separate the names, "To Mr. John Smith and Dr. Mary Smith."

If it is a casual letter you could simply write "To The Smiths"

I am guessing the original form of address you want to modify is "To Dr. and Mrs. John A Smith." But, this is an increasingly archaic form of address and is not applicable in the case of the wife being the doctor or professor nor can it be used when the wife has not taken her husband's name.