I believe that every soul reincarnates. That ech life is another lesson that the soul learns... that this has been happening since the beginning of time, and that some souls are very old and wise...

Sometimes Soul's split, when they have reached a point that they can no-longer grow with knowledge, and become two, new souls, with limited knowledge again... this is not necessarily good. For the split soul will always be searching for it's other half... and depending upon the circumstances that caused it to split... they will stay together for their following lives... or other things happen and the cycles get messed up, one dying more often than the other: becoming wiser and then the soul left behind feels lost... or in following lives they never meet and always feel empty; or even in the following lives, they are near each other but not truly together... therefore becoming frustrated...

It doesn't matter really... in the end... I think... they eventually find each other and become one again.

*shrugs* I have reason to believe this although you may all think it's fantasy...