I'm writing my professor. She has two surnames. So far, I've been using only her first surname; however, I'm curious whether that's, generally, how I should address a professor with two surnames. I don't know whether the addressee's culture affects the answer, but the professor is from Spanish South-America. (I've noticed that some Spanish people use only the first of both last names, but most English people who take two names ask others to use both names.) Also, I don't whether familiarity matters but we communicate frequently enough that I begin my emails with 'Hi Dr...' instead of 'Dear Dr...'. 

Thank you