How should I politely address an email to two professors, one who holds a PhD and one who does not? I am planning to request an override via e-mail for a class which is taught by two professors; one of them holds a PhD, while the other hold a master's; I have never met either of them. Usually my first line would be something along the lines of:

"Hello Dr. X, Dr. Y"

However, I feel like if I say

"Hello Dr. X, Mr. Y"

it would seem a little weird. I don't want to be over the top formal, but I still want to be polite. I've thought of using "Pro. X, Prof Y" but I'm not sure. How should I address them?

My location is the U.S.
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.