Well tbf they were babies. It's not like they remembered each other anyways. And if Namsoo hadn't found out about Hyeongoh he probably would've either never met or met later in life as adults which would be a different approach than this story. I think you shouldn't separate siblings that are old enough to have a sense of awareness and that rely on each other. But in Hyeongoh and Namsoo's case, they were practically newborn babies. You can argue they had some kind of a bond or whatever but both of them never thought about the other throughout their lives as they didn't know the other existed and at least in Hyeongoh's case was perfectly fine. Namsoo would've also probably been perfectly fine if he had lived a happier life if not for the unfortunate circumstances that happened to his adoptive family.
Also, if there was a rule that people couldn't adopt siblings separately, do you realize how many kids would never get adopted at all because not everyone can afford multiple children?
parents who are planning to adopt shouldn't be allowed to separate siblings