Entities do facilitate personality splits.
They do tend to give people a dual nature, because the wantings of the entities are superimposed onto the person's own wantings. Sometimes the wantings reinforce each other, as in the case of the beer drinker who catches an entity in a pub and afterwards wants beer more than ever. They make it more intense, and more difficult to deal with. However, for someone who already has a tendency to split personality, catching an entity is likely to make things much worse. Entities cause sudden, abrupt impulses that the patient doesn't recognise as coming from a foreign parasite but believes to be his or her own.