When I studied embryology I learned that it is possible for a fertilized egg (a blastocyst) to split into two embryos, forming identical twins, and that this can happen as late as seven to nine days after fertilization. If that is the case, then how can individual human lives begin at conception?
Human souls are pure spirit. Although there is no way for science to measure this, it would seem to me that there is no reason why two human souls cannot occupy the same blastocyst. In fact perhaps it could be the very fact that there are two human souls present in the same blastocyst that is one of the reasons the blastocyst separates. Since the human soul is the form of the human body—it is what makes a material form for a human to be a human body (just like a dog soul makes a material form for a dog to be a dog)—then if two human souls are present in the same blastocyst, that may be one reason two human bodies “emerge” from the blastocyst.
Philosophical speculation aside, all we really need to know and affirm is that human life begins at conception. We don’t have to understand exactly how that happens, at least at this point, to be able to affirm that truth.