The Bean and the Peanut are usually pretty tight. They like each other a lot and play together like peas in a pod. However, I sometimes wonder if they have an actual biological need to scream at each other every so often until one (doesn't matter which, sometimes it's both) of them is crying. There doesn't seem to be anything I can do to prevent it. If I see it coming and separate them, they just wait till the next available opportunity. It's like a necessary, inevitable letting off of steam.
I have learned to just send them outside till it's over. Let the neighbors wonder. "What in hell is that racket? Where on Earth is their mother, and why doesn't she do anything about it?"