Are you 100% sure they are not play fighting? Some cats are just louder than others. I have 2 new kitten fosters and they wrestle to the "death" and they scream, but they are loving every minute of it.

Is it like sibling rivalry? Are there at least 2 litter boxes, 2 separate food and water dishes? so they need not share if they didn't want to.

Are they both neutered?

You could always try the feliway plugins. They are meant to bring calm to the cat's area.