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.

In the US, it's perfectly fine to say "Hello (or dear) Professor X and Professor Y", or something like Dear Professors. Another widely-applicable option is to avoid names altogether -- my favorite is simply "Greetings."

If it is an option to send them separate emails, I would do that and address both as "Professor." As someone who holds a Ph.D. and worked in academia with colleagues who have a Ph.D. and others who don't, I would say that no one would be discomforted by being addressed as "Professor" even when they don't have a doctoral degree. Since they are teaching college level, it is appropriate to address them as such regardless of their degree.

PS: I am assuming this is for US colleges. Although I think the academic etiquette practiced in some other parts of the world might be similar, I can't speak for it informedly. And you get an "A" from me for caring to ask this question :)