In Destiny of Souls, Dr. Michael Newton talks of the different kinds of soul relationships, but disagrees with the concept of Twin Souls.
First, he counters that notion with his discovery of what he terms "parallel lives," in which a soul may choose to split into two or more parts and then experience life in simultaneous incarnations. According to his explanation, a soul in parallel lives differs from what I would term a Twin Soul in two ways: (1) The two parts often don't have contact with each other in that lifetime; and (2) There is no drive, no deep yearning, to reunite. Dr. Newton further explains that very few souls choose this kind of incarnation.
Second, he states that at the moment of creation, we are individuals, not a part of some other soul. As he writes, "Each energy particle is unique in its own right and created as a single entity." He denies the idea that Twin Souls are two halves of one soul, created to be united again at the end of our incarnations.
What's my take on all this? More on that to come.