I'm not a parent, but I have studied psychology for many years.  My first question would be, "What are they fighting about?"

Good communication is always the key to fixing anger if it's situational.  The problem is that fighting can be a sign of so many things -- depression and bipolar disorder are two examples that many parents never even think of (nor want to think of).  However, if they're only fighting between each other and they get along relatively well with everyone else, a chemical problem as a root cause is unlikely.  Being so close in age as they are, I would suspect plain old sibling rivalry.

Have you tried talking to them individually to find out what the problem is?  Perhaps a family outing with the three of you would help build some bonds, perhaps something more team-oriented rather than competitive, like camping or hiking, that inspires connection to the others around you?  Be sure, too, that one boy is never shown preference by the parent over the other.  That can lead to serious self-esteem issues later.

On the bright side, many young boys will outgrow the need for fighting as they get older.  If they're taught good communication skills as children (e.g., learning how to state how they feel and taking responsibility for those feelings rather than attacking and blaming the other), respect for one another naturally follows.  Of course, this implies the parents have also learned those skills.

I wish you all the luck with this one!