I was perplexed to see that you advised people to address envelopes using the title "Dr." only for physicians, but a Ph.D. to be addressed as Mr., Mrs. or Ms.
I see a contradiction that I hope you will correct. On your web page, you refer to yourselves as both "Drs. Dave and Dee" and "Dr. Dave and Dr. Dee." However, Dr. Dave has an M.D. and Dr. Dee has a Ph.D. Please clear this up by acknowledging all of the Ph.D.'s who would be disheartened to read your advice, that is, to drop their own Ph.D. in deference to the M.D. This does not make sense to me.
Dear Perplexed, Ph.D.
Please note, the title "Dr." for physicians had referred to the traditional addressing of envelopes, so there was no contradiction regarding the salutation Dr. Dave and Dr. Dee.
Salutations, greetings, and introductions use Dr. for someone with a Ph.D. For example, when Jane Doe, Ph.D. is being introduced, she can be referred to as Dr. Jane Doe. Letters can use the salutation, Dear Dr. Doe.
As previously noted, addressing the envelope with Dr. for someone with a Ph.D. is fine nowadays.