We had gotten them after they were fully grown, they did not grow up together.
We had a big lot and there was plenty of room to run, and they just loved to play together. We had an old throw rug that they glommed onto, tugging and pulling from either end. As one would slack up a bit with their tug-of-war, the other would grab just a little bit more. Well, they slowly got closer and closer to each other, and after a few minutes their noses were quite close to each other.
Well, neither would give in, so, shortly, they got so close that one of them chomped the tender nose of the other. That was all it took! And unintentional offense turned into a MASSIVE dog fight!
It's a good thing that there were two humans around to grab a dog before it got REALLY ugly!