If a letter needs to be sent to a couple both are doctors How would you frame the salutation Will it be Dr. x Dr. y or Dr. Dr. followed by surname or Drs followed by surname?

If you are giving both first names, you should use separate titles, e.g. "Dr. Pierre and Dr. Marie Curie"

Please note that f.i. in Dutch Drs is a completely different title from Dr (Drs being a Masters degree, Dr being either a doctorate or a medical title).