We had two bitches, sisters from the same litter. They were both spayed at around a year old. But from around their first birthday for a while (can't remember exactly) they were always grumbling and snapping at each other, trying to decide which of the two was the dominant bitch. They actually came to a compromise in the end - Daisy was the top dog in the house, and Phoebe was the Master of the Hunt when they were out. But once they both knew their places, they got on beautifully and were always very affectionate with each other. It depends I think on the temperaments of the bitches - if they both think they should be top and refuse to back down, I don't think the bickering would ever stop.