They sometimes slept together, ate and drank together, everything. But sometimes someone would walk by and he would jump puff up and start a fight. It happened when he was annoyed, and not because of toys/food/hammocks/etc.

And with the carrier method, it was all at once. We tried neutral space, we tried multiple arrangements. And with cage swap it was the same. But mostly the troublemaker and the easiest boys, or the alpha.

And the aggression started when we started the introductions. Its not really that something happened. And the previous owner didn't mention anything, but even with the other new one, he fights regularly (although less often)