first off, I believe we ALL have a dominant soul "essence", an always there always ON spirit to which we return (although, truthfully, we never really LEAVE as the energy is always running from soul essence to body and versa meets vice)
as a result, we can (in my opinion) inhabit multiple bodies at the same calendar time; there might be two incarnations of a specific soul essence, or three, or four (maybe more)
but in that case, EACH of the incarnations would all have the EXACT same "past lives"
some people have theorized that certain individuals (often, spiritual teachers like Jesus or Buddha) have thousands of souls wrapped up within them; I don't agree with this
no matter how magical or evolved, these Masters come from an essence just like you and me
and if you share a past life with the Buddha, you share ALL past lives with the Buddha
he isn't a composite of hundreds or thousands . . . he is one person, and comes from his essence and no one else "time shares" his body with him
soooooo . . . just a quick note
but (1) -- if you share one past life, you share every past life
(2) you can live in more than one physical body in the same chronological period, but every incarnation is connected to the dominant source essence
and, (3) there is no blending of souls
the body is a vibratory "expression" of the dominant essence, no matter how many lifetimes
there is always a one to one relationship between physical incarnation and soul essence