How should I address my two supervisors (one man, one woman) in an email?

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.

This may sound flippant but I’m being 100% serious:

You’re doing a PhD in STEM and addressing your supervisors. The customary form of address is “Hi” or, if you have a very formal relationship with your supervisors, “Hello”; not “Dear Prof. Dr. X” — even in the traditionally hierarchical German system.

Starting an email to your supervisor with “Dear Prof. Dr. X” is distinctly weird and risks coming across as off-putting. If you’re not on first-name terms with your supervisors, simply leave off the name entirely from the salutation (i.e. write just “Hello”, not “Hello Xavier”).

In addition to @PhilYardman's point (which is correct) you also need to consider how directly your email addresses whom. This beats both hierarchy and gender: the importance in the style link in @PhilYardman's answer is evaluated in the context of the email.

• The ones who are primarily addressed (To) come first and are always greeted.
  So, if you primarily write to your secondary supervisor and "only" keep your supervisor informed of this, the greeting starts with "Dear Dr. Second-Supervisor"

• Those who are only informed (CC):

  • I name them, but usually put that greeting into parentheses. This reminds everyone that more people are reading this email, while also signalling that the ones in parentesis are only notified.
  • It would also be acceptable to not include them in the greeting.

• If there are further people on BCC, they are not named (that would defeat the purpose of BCC), but you may use a generic greeting - depending on the context and content anywhere between "Dear all" and "Sehr geehrte Damen und Herren" [Dear Madam or Sir - but the German version is in plural], or a group address (Say, "Dear supervisors")

• I'd adapt the level of formality ("Sehr geehrter Herr Prof. Dr. med. mult. Supervisor" - "Sehr geehrter Herr Prof. Supervisor" - "Lieber Prof. Supervisor" - "Lieber Herr Supervisor" - "Lieber Vorname") to the customs at your institute and to the likes of your supervisors. This varies a lot between fields, institutes and people...
  "Prof. Dr." sounds too formal to me for the STEM fields I work with - but e.g. in the medical field it is AFAIK quite common (and there the position in the hospital would be attached as well "Chefarzt Dr. med.").