Another thing you can do is to allow the children to work it out themselves. If one of them does some damage to property or person during the fights, you can hold that person responsible for the damage and provide consequences for that. This is different than trying to provide consequences for the fighting behavior. The ordinary bickering and fighting I have been talking about is one thing. That is something they will outgrow as they mature.
Real animosity is something else.
If siblings truly dislike one another, or if one hates the other and is constantly picking on him out of malice, that is a different problem. It is a problem too serious and too deep to be cured by simple behavior modification techniques. However, it is possible that if from the very beginning you take the laid-back approach and refuse to let yourself get involved in their arguments, you may help prevent that kind of animosity from developing. You can be on the lookout to see if the episodes of fighting are mutual with both participating, or if it is just one child picking on the other with no instigation from the other. These are the kinds of fighting which may require your intervention.