However, cases presumed to be of the many-souls-one-body relationship frequently turn out to be instances of split personality. They are not exceptions to the one-body-one-soul equation. In split personality some dissociated nexus from the psyche of the same soul gets possession of its body and seeks separate and exclusive expression. It is not another soul or spirit but another group of impressions (sanskaras) of the same soul or spirit. In split personality each homogeneous nexus of the psyche takes possession of the body for some time, to make room for other nexuses of this or previous incarnations. These other nexuses are qualitatively different and are incapable of being accommodated in the first nexus; so they seek alternate expression instead of simultaneous adjustments.
In the realm of literature, a famous example of dual personality is that of Dr. Jekyll and Mr. Hyde. This is an example of two sections of the mind dominating one body, and not of two souls getting linked up with it. However, it remains true that in some instances two or more souls can use the same body. When this happens, there is generally an alternation between their regimes. Simultaneous use of the same body by different souls comes under a special exception, where the different souls have to be in perfect harmony with each other. The harmony between the souls has to be so complete that one and the same bodily action or experience equally meets the needs of all the souls concerned.
Some advanced disciples are in such complete harmony with the master that the master can, if he so wills, sometimes “overshadow” the ego-mind and the body of a disciple, without supplanting his soul. Such overshadowing may take place for the working out of the wider plans of the master. Instead of replacing or wiping out the individuality of the disciple, it implements and amplifies his individuality. Here the disciple and his master are so merged into each other that they both find equal fulfillment through whatever the disciple achieves through his bodily existence.
-Beams, p45