How should I address my two supervisors (one man, one woman) in an email? I am currently doing a PhD. My professor, which is my first supervisor, is a man. My second supervisor is a woman and works for my first supervisor. The etiquette and the courtesy, at least in my country, requires to greet the female first, e.g.:

Dear Dr. Female, 
Dear Prof. Dr. Male,

This sounds wrong to me, since my professor is my main supervisor. Can I greet my professor first without violating etiquette?
My PhD position is located in Germany; my research field is mathematics and I write everything in English.
Hierarchy beats Gender in Germany
In a professional context in Germany, hierarchy beats gender, at least according to the Knigge, which is an etiquette guide of nontrivial influence. This goes as far as to be gender-blind. Only rank is important.
So you would address the highest ranking person first, in this case, your professor.