Our pair of girls (unrelated, 14 months and 11 months) are fine together. The odd bit of handbags-at-dawn initially, but we always stopped it before it got more than play-fighting to make sure it didn't become a habit. They still play-fight, but they're very attached to each other now and don't like going anywhere without the other one.
The only pair of dogs locally that I know of having problems together are a boy and girl pair of cockers. The owners knew the female wasn't keen on other dogs but they still got another dog, a male, and did it without checking first that the female was OK with him. After over a year, the bitch has settled down a bit, but there's been a lot of problems with them. Both were neutered, so nothing to do with that - just that the female wasn't happy being with other dogs.