Imagine that you’re writing a formal email to two people who work in the same organization. In other words, it will be sent to two addresses. One of the addressees is a woman, the other is a man. What will be the greeting in this case:
“Dear Madam / Sir” or “Dear Sirs”
If you know their names, write Dear Mr xxx and Mrs/Miss YYY.
If you don’t know their names but know that one is a man and one is a woman, write Dear Sir/Madam.