How do my parents I want them to stop fighting?

If you feel it's too overwhelming to tell them both at the same time, try to do one-on-one conversation with them. Go to the one you are the most comfortable talking to. Don't do it when they are about to fight or directly after a fight, tell them when they don't seem stressed and don't seem to have a busy mind. Don't only talk about how it makes you feel, but also about your concerns. Don't try and separate them while fighting, you might become a common target and don't want to get in the middle of their argument. Maybe you won't need to tell both of them as they would realize how it's affecting you and discuss it among themselves. If you think it wouldn't be the case, then make sure to talk to them separately.