I have 2 boys who are also 2 years apart. They are very close, but like any two people who are together as much as they are, they occasionally get into fights. If it sounds like they'll be able to work things out without anyone getting hurt, I leave them alone. (Being able to negotiate and compromise is an important life skill that I want them to learn.) But once in a while, tempers flare and they need me to intervene before things get too out of hand.
Here's what I do in those situations:
1) I send them to time-outs in separate rooms so that they can cool down. I leave them totally alone for about 5-10 minutes (my kids are teenagers; if they were much younger, I wouldn't wait as long.
2) When they are calm enough to have a rational conversation, I sit down with them one at a time. First, I actively listen to the one I'm talking to tell his side of the story. I only interrupt if I need clarification.
3) Once he knows that I have really heard what he has to say, I ask: "Now what could YOU have done differently that could have prevented this?" (I'm hoping for an answer such as "I would tell him how I felt in a nicer way." Or "I would put myself in timeout so that I could calm down." Or "I would stop playing until he's ready to play by the rules.") Sometimes, they say "I wouldn't do anything differently. It's all HIS fault." When that happens, I may try to help him see things from his brother's perspective. I also say things like, "You can't control your brother's behavior, but you can control your behavior. I want to know what are YOU going to do differently next time. Remember, I'm going to have this same conversation with your brother. "
4) Sometimes, I bring them together to do a little negotiating. "I'll do ________ if you do __________."
5) We always end by appologizing, but it's genuine by that time, not forced.
By consistently following this little plan, I have had to use it less and less frequently, because they have gotten better skilled at solving conflicts on their own. The most important part, I think, is really listening to what each one has to say and not taking one side or they other.