Once they are separated the pecking order changes, and it can trigger fighting, especially if it has been that long. In my flock, if a young cockerel grows up with another or a grown rooster, they usually get along fine, until the young once challenges the older one. If the young one or the one who has been gone act submissive, they might be able to get along. My head rooster went through a molt, and the new guy ran him off into the woods and wouldn’t allow him back.