How should I politely address an email to two professors, one who holds a PhD and one who does not?
Given that they're professors, you can just use "Prof."

Having said that, don't worry too much about salutation. I'm sure lots of people have received emails calling them "Dr." when they don't have PhDs, or "Prof" when they aren't professors. Heck, I've even seen emails calling someone "Mr." when they're actually female, or vice versa. We learn to ignore the honorific and concentrate on the email text.