So, let's just say this happens at your house, too...
You're upstairs and you hear screaming downstairs. You know it's your two children fighting again. One is suspiciously quiet while the other is screaming at the top of his/her lungs. What do you do?
Do you always run down and address it?
Do you discipline both no matter what?
Do you let them figure it out on their own as long as no one is getting hurt?
Or some other bright idea I haven't yet thought of?