As i believe the soul is a non-localized phenomenon, and given that all things change over time; I have trouble believing that soul mates are singular entities. When similar 'souls' meet, both being emergent patterns of causality and form, there's a chance for them to take a journey together, growing and changing at the same pace to the same rhythms. Soul mates might not always remain on the same path--but that one-ness, once experienced, means that they carry a large chunk of the other for lifetimes to come.