As a rule we equate one body with one soul. The equation of one body implying one soul holds true, particularly in the higher stages of evolution. However, the exceptions to this general rule are also interesting and noteworthy. In the frequent cases of obsessions we have an example of one and the same physical body being used alternately by different souls. Some departed spirit or spirits can occasionally take possession of the gross body of an incarnate soul and try to use it for obtaining experiences of the gross world. When this happens too often, the general equation of one body with one soul is seriously disturbed. We have here only one gross body but many souls who are using it, in succession or alternation.
Just as one body can get linked up with many souls, one soul can get linked up with many bodies. This can occur at the earlier stages of evolution. There are group-souls. Each group-soul gets linked up with numberless physical forms as in the case of blades of grass. When the group-soul gets more advanced in consciousness then it can get connected with one form instead of with numerous forms. Group-souls are an exception to the one-body-one-soul equation.
But whether souls come under this equation or fall outside of it, their externality is due to imagined separateness. Even a group-soul is external to another group-soul in the inner spheres, and is as illusory as any other separative soul. In truth, there is only one cosmic soul which gathers experience through all the different forms, sometimes appearing to separate itself into group-souls and at other times into souls which come under the body-soul equation. Group-souls imply a one-soul-many-bodies relationship, which is the opposite of the many-souls-one-body relationship found in obsessions and possessions.
-Beams, 43