My first job after being at home for a few years was at a college and the only other person with children complained that her children hated each other. In fact her oldest had asked from the very beginning when they were going to be returning her baby sister, she's been waiting ever since. lol.
So why is it this kind of behavior has not darkened my door? Well aside from the fact that I have awesome children who are perfect in every possible way, lol, I suspect it has to do with what they see. Friendship is a very important thing to me and I have always tried to be a good friend, not just to my own sister but to others. When my oldest was a baby and toddler I was a freshman in college and we spent many a day hanging out at the sorority house. He loved it.
Before long I discovered that he had become very kind and observant of people and behaviors. The first time he was babysat by a friend with children (she had an older and younger), she just raved about wonderful he was. She could hardly believe how observant and smart he was. I asked her to explain, she told me that her baby would throw a tantrum every time my son wanted to play with a toy, my son responded by allowing the younger child to play with it and when the child lost interest he would resume playing with the toy.
I have never subscribed to the thought of allowing a first child to be selfish and then teaching them to share later, that's crap (pardon my language). Human nature is resistant to change and sharing is not something that comes naturally. If you teach a child to share from the beginning then it won't be a culture shock that builds resentment later. Learning to share is the first step fostering friendship.
My baby is 1 and when she hears the word share she immediately gives what she has to the nearest child. This concept is continually reinforced by her siblings and it helps to reinforce the concept for themselves. We all want others to share with us and it is important that we do the same when it is our turn.
So, does that mean they'll always share? No. Does that mean they'll never fight? HAHAHAHAHAHA, NO!! But you said. . . friends don't always agree and they fight sometimes. You just need to figure out which battles they need to handle alone and which you need to get involved. Good luck but here's a quick tip, they can handle more things on their own than you think if you give them some space. (your job is to make sure you always know what's going on even if you say nothing).