Some traditions may talk of sheltering many souls in our body life vessel.  I prefer to speak of hearts, one has many hearts. Where do these hearts come from? pretty much from anywhere in the cosmos.  And they are put, in an arrangement of mix of hearts , into one person or sentient being alive on earth. A truly well lived life, is when all souls or hearts are liberated at the end of the life cycle. Life itself being eternal, I am not sure when the mix of hearts becomes pure enough and good at working together enough that some may start their own rebirth elsewhere before even the person , the original person, is dead. 

P.S. I suspect some hearts can actually be put to sleep during some life time for certain lapses of time....