My three sows have been living separately for a little over a year. The last time I attempted cage them together, Pinni and Bertie harassed Willow like a pair of thugs: taking turns beating on her, chasing her away from food and ousting her from the best sleeping spaces.
When they're outdoors, though, it's a different story. They can live peaceably together for several hours; nothing more than the occasional tiff. I caught all three of them snuggled together under the stool yesterday. I managed to snap this photo of them today. Proof! Look! They can all get along. Not only are they not fighting, they're actually sitting near each other. Amazing.
As soon as they cross the threshold, the rules change. We accidentally put Pinniped and Willow in the same cage yesterday. No sooner than the two of them emerge from their cozies did the teeth-chattering commence. I snatched Willow out of there before the hair pulling started.
The outdoor pen seems to be completely neutral territory. They don't even spend time re-establishing pecking order. It just is. Even floor time in the kitchen isn't like this. I wish I could bottle whatever it is about outdoors and use it while they're inside!