From what I read, they are born female (opposite to clownfish) and turn into male. If you observed closely, male is bigger and has more intense coloration and thicker, while female is pale and slimmer. Because my 2nd one was also a bit mature, IMO, it was attempting to become the dominant one. That's why it took longer to be submissive to the other.
So the smaller the better.
They are swimming together now