To begin with, I think that each person only has one half of a soul, and that your soulmate is a half soul that fits perfectly with yours. We have best friends, and enimies depending on how well our half souls match.
Now to the Soulmate part...
I came to this conclusion when I noticed how I felt whole when I was with my ex-boyfriend(he moved, hence we broke up). There are many many sayings that coincide with this; two becomes one, when someone "melts" into the other.
We feel we are one "being" when we are with someone who matches completely, or pretty damn close to it. We are soulmates with the ones that match exactly. We know they match completely when the feeling of oneness becomes adictive.
Another part to my theory is that you can have more than one perfect match. There are so many different beings out there that hold a half soul. Not to mention that bisexuals like myself could have a soulmate in both genders, or in just one gender, it doesn't matter. You may even have an otherworldly soulmate, maybe a creature that can live only in the astral plane, like a dragon or a unicorn. Though that means you will probably never have kids, so be it.
There are some people who would totally disagree with what I have written here, but this my "theory."