The etiquette and the courtesy, at least in my country, requires to greet the female first
This answer will be a little bit on the philosophical side (probably too much so for some people’s taste), but I’d like to suggest another way of thinking about the question. Instead of asking what is tantamount to “how can I best conform to what my society expects of me in email greetings?”, you can turn the dilemma around and ask “how can I help dismantle antiquated social norms that assign different genders to predetermined roles, and push society ever so slightly in the direction of being more rational and fair?” In other words, put the focus not on being the best follower, but on being the best leader. Granted, the particular social norm in question is a harmless one, but the point is that it’s part of a larger pattern of gender-aware (or outright sexist) norms which are not all harmless.
If you like the idea of looking at things that way, just write the greeting in the way that seems most logical to you based on the content of your email and your relationships with the two co-advisors.
Of course, the specific dilemma in your question is so trivial that it doesn’t really provide much of an opportunity to be a leader. Whatever you do in this specific situation, I doubt anyone will even notice it, let alone know to interpret your behavior as a conscious attempt at shifting social norms (although if they happen to ask you about it, you can certainly explain the thinking that went into your decision). But I’m mainly suggesting this as a mindset to adopt in similar situations that you may encounter in the future, some of which may be more consequential. In other words, the current situation can be used as a kind of training or practice opportunity to get into the habit of challenging stupid social norms — particularly in a low-stakes environment where no matter what you do, it’s almost certain that nothing bad will happen.