1) Both doctors, he is called Alan Smith and she is Jane Smith. The salutation would be "Drs A and J Smith", or sometimes simply "Drs Smith".
2) He's a doctor, she's not: "Dr and Mrs Smith", or "Dr A and Mrs J Smith".
3) She's a doctor, he's not: "Mr and Dr Smith", or Mr A and Dr J Smith".
There may be a doctor on here who will correct this, or certainly have an opinion on how they would expect to see the salutation.