I've kept rabbits since I was four. Im coming on fifteen now, and I recently got rid of my remaining rabbits. I gave them to a friend of mine.
During those 11 years, I've read so much about rabbits, I was almost like a walking encyclopaedia. I can't remember as much nowadays, but I certainly know the answer to that problem >.<
Check their Genders, If one's male, and the other female, I think you can guess why they act like that. Even if they are brother and sister.
If they're both Male, Seperate them. They'll end up fighting. Males tend to do that.
If they're both female, they may calm down. But my female rabbits never seemed to get on.
I really just suggest you seperate them. Unless they're both of different genders. You could seperate them, But you could get neuter the male, or spay the female. I suggest you spay the female, as it lowers the risk of Cancer for her.