Hard to tell what pees them off, but I just rescued 2 males that have been together from months old, though not related. The lady said they were fine then started fighting after 2 years together. I had to separate them as soon as I picked them up. The little one was getting quite the licking from the big one. Never came to her to separate them
They seem fine now after 3 months apart. Though they know each other is still around, but now there are many more too.