My kids, being 3.5 and 1.5 years old, are prone to fighting with each other. Nothing major and nothing that can't be fixed with a hug and an apology, but sometimes it seems like whenever one picks up a toy, the other tries to get it, and when the other starts to do something, the one wants to be involved without being welcomed to do so.
I don't like to play referee all the time. In many cases, they are able to work it out, either by one of them getting distracted by somehing "better" to do, or by The Boy behaving diplomatically and giving Meatball a toy, saying "here, you can play with this!" or even *gasp* sharing what he has if it is something like letters and crayons.
But I also want to discourage both of them from hitting each other out of frustration. They'll learn eventually because they figure out that if you hit someone, they're liable to hit you back, whether it's a good idea or not. So maybe that, too, will work itself out, but probably not. Given that boys in particular and children in general are prone to smacking the crap out of each other for funsies.
Please, share with me your tales of sibling rivalry and toddler warfare.