During the Spirit Awakening Ceremony, those who have two Spirits Essences are known as having Twin Spirits. People with Twin Spirits usually has Innate Full Spirit Power.
There are exceptions like Huo Yuhao. He has Twin Spirits but he acquired his second spirit after the Spirit Awakening Ceremony, from a intelligent Spirit Beast, it's a long and complicated process.
It is a rare ability that most people don't have. With Twin Spirits, the Spirit master can have 18 spirit rings, nine per spirit essence, and 19 if they become god. Which is still something that has only ever occurred twice.
In soul lands 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.
A Spirit Master with Twin Spirits can't use them at the same time, but there are exception like Xie Xie or Xiao Xiao.
There are Twin Spirits weaker than others, for example, Xie Xie has the Light Dragon Dagger and Shadow Dragon Dagger an he has to obtain Spirit Rings for both of his Spirits whenever he hit a bottleneck preventing him from reaching the next rank.