If that is so, tell me exactly what is taking place in an identical twin embryo? Can one cell house two souls?
Science is absolutely firm on the truth that an individual life begins at conception. Only one of two possible answers must be the truth about twinning: 1) within the single cell at fecundation (conception) there must exist the genetic quantity of two or more individual beings, or 2) something is in the first single cell of twins that will split the genetic material soon to express the two individual beings, some anomoly that will act to 'split the road' that is both of them at the start.
The point to focus upon is both twins originated in the first single cell at fecundation/conception. Every individual human being has its beginning as a single cell, the first age of its individual lifetime. If, within the first five days of fecundation, a second identical individual, or even a third individual presents and grows along the continuum of individual life, each of those individuals had their origin in the first single cell of fecundation. There is no other truthful conclusion possible. Within the first single cell at fecundation are all the individuals conceived in that conception.
Since it is a fact that an individual lifetime is a continuum that begins at fecundation, at conception, it is glaringly paradoxical to arbitrarily remove any age along that continuum in an effort to prove a later start to the continuum (kind of like saying you hold a single strand of rope, but then you cut the rope a foot from your left hand and assert that the after-cut rope is the exact same length rope you held before you cut a foot off of it AND pointing to the short tag of rope on the floor, assert that that piece of rope was never a part of the piece you were left holding AFTER you made the cut). If a person tells you that he or she originated at 18 weeks from conception, or originated when she or he took their first breath, or originated when his or her brain first had a thought, or originated when his or her heart muscle first contracted, or originated when her or his gonads first functioned, just remember, the lifetime of every individual human BEING begins at their unique conception/fecundation and to choose some other point to believe the continuum begins is absolutely arbitrarily illogical, not based in science or truth.
So, yes, with first cell division, at least one HUMAN BEING is present on the continuum of at least one human lifetime. With identical twinning (or more for that matter) all conceived individual human lives of that single cell conception are present.
If you've ever known any identical twins well, you know that they are not really "identical," despite their common beginnings.