I've finished the wonderful, groundbreaking book, Journey of Souls, by Michael Newton. Wow. It really brings clarity to our purpose here on Earth, and what happens between our reincarnations. If you are interested in this sort of thing (and who isn't curious about the purpose of their life?), order this book!
That said, I was hoping to find more answers about Twin Souls in this book. The author did mention the concept of parallel lives, but my sense was that this isn't quite the same thing, because the split souls didn't seem all that interested in finding each other. Parallel lives are taken, however, for the primary purpose of speeding up the learning process.
Dr. Newton did mention soulmates quite a bit, and after reading his definition of soulmates (quite near the end of the book), I'm wondering if Twin Souls and his idea of soulmates weren't confused. It would be interesting for a hypnotherapist to explore this area.
Perhaps someday we'll know more. . . .