Dear Halaliel,
Fascinating idea! We can never know for sure, just as we really can't know for sure if we really have had previous lives, or are they going on simultaneously, or are we just resonating with another soul, so we feel like "one".
I did look at my guides and feel that what your RM said COULD be a possibility. As I am studying Sacred Contracts right now and looking at archetypes, I can see that my three major guides could be aspects of my own soul.....
My RM has told stories of travelling psychically to other parts of the world to heal a part of her self that was actually another person living currently...
As others have said, the possibilities are endless, and isn't life a great adventure?
Love,
Feather