When I first got my cats, they were close as could be. They had each other, and they had me, and all was right with the world. After a while, we moved in with two other cats, both about as unsociable as could be, and my girls still had each other and me, so everything was fine as long as nobody pissed off the other cats. We moved back to my childhood home for a few years, where my girls were kept apart from the two resident cats in order to avoid squabbling, and that mostly worked.
When we moved back to Vermont, though, the dynamic began to change. We had a sociable cat to share the new house with, as well as another cuddly human (my now-ex-husband). I'm not sure if Kira and Maeve decided that there were things in life worth fighting each other for, or if they got used to having someone to look down on (my ex's cat was the baby of the group, and utterly submissive), but after my ex and his cat moved out, my girls were suddenly not friends with each other anymore.
They'd fight over cuddle time with me, they'd fight over prime napping or bird-watching spots, they'd fight over darned near anything, despite the fact that the house is big enough for them each to have several rooms worth of territory that don't overlap with the other's. I played referee every so often, but knew they had to work it out for themselves.
It's been a few years now, and while things aren't back to the way they used to be, they're better. Kira still throws a yowling fit if Maeve claims my lap first, and they don't cuddle or groom each other the way they did when they were little, but they can be next to each other without it ending in hissing.
That said, there's still some tension. This may look cozy, but those paws are engaged in some passive-aggressive behavior. One pushes the other to say, "this space is mine," and the other pushes back to say, "no, it isn't." Sometimes this is as far as it goes, but other times it'll progress to paws on faces, and then one or both will realize it's about to get dangerous, and beat a hasty retreat.
Still, they're both on my lap, and they're not hissing at each other, so I call it a win.