During the Spirit Awakening Ceremony, those who have two Spirits Essences are known as having Twin Spirits.
There are exceptions like Huo Yuhao. He has Twin Spirits but he aquired his second spirit after the Spirit Awakening Ceremony, from a inteligent Spirit Beast, it's along and complicated process trust me.
It is a rare ability that most people doesn´t have . With Twin Spirits, the Spirit master can have 18 spirit rings, nine per spirit essence, and 19 if they become god. But in history there has been cases where the body can't take the spirit rings added onto both spirit essences and has killed the user. It is also dangerous to the body if the two spirit essences posses opposing attributes, if they clash within the body.