Two people got drunk and started fighting with knives. How do I convince them to stop?
The best course of action in this situation is to immediately call 911 or local emergency services. Attempting to physically intervene or negotiate with armed individuals who are in a state of intoxication and aggression is very dangerous and shouldn't be attempted.
Ok, but what if I'm worried about one of the individuals who is fighting? Let's say that one of them is my brother. What should I do then? I can't just watch him die.
I understand that this is a very difficult situation. At this point you could try to defuse the conflict verbally from a safe distance.
I would still advise you to not physically intervene while you wait for emergency services as this could get you in trouble as well, 
causing two deaths instead of one which would be even worse.