In the physical world, we regard a person as a single entity. Spirit individuality however, is not separated in the same way. There are for example, affinities, two kindred halves of the one soul, and sometimes they incarnate at the same time.
There is a group-soul, an over-soul, or to use a simile--a diamond--with greater individuality. Aspects of this greater individuality incarnate into this world for experiences that will add a luster to the diamond when they return to it.
There are also aspects of one individuality, or extensions that exist. Only an infinitesimal part of the whole individuality can be manifested in physical form on Earth.