Perhaps, as the conceptus is beginning to divide into two or more separate humans it receives a second soul or more.
My guess is that you would like a "scientific answer," but science is only beginning to learn about the time period very shortly after conception.
Your destiny, from day one { Embryos differentiate early, aren't blobs} Nature ^ | 8 July 2002 | Helen Pearson
Our body plan is being defined in the first few hours of life.Your world was shaped in the first 24 hours after conception. Where your head and feet would sprout, and which side would form your back and which your belly, were being defined in the minutes and hours after sperm and egg united.
Just five years ago, this statement would have been heresy. Mammalian embryos were thought to spend their first few days as a featureless orb of cells. Only later, at about the time of implantation into the wall of the uterus, were cells thought to acquire distinct 'fates' determining their positions in the future body.
[snip]
What is clear is that developmental biologists will no longer dismiss early mammalian embryos as featureless bundles of cells - and that leaves them with some work to do. "I believe in the new philosophy," says Tom Fleming, a developmental biologist at the University of Southampton, UK, "but there's a lot of detail yet to be understood."
Perhaps, as the conceptus is beginning to divide into two or more separate humans it receives a second soul or more.
We are certain to not agree on what a soul is. Since I do not believe it is possible to separate "consciousness" from the meaning of soul, whatever you believe a soul is, to say something has more than one soul would mean it had more than one consciousness, which is absurd.
My guess is that you would like a "scientific answer," ...
Hardly. It is not a question science can address. The consciousness is subjective, as we experience it. Science deals with the objective and demonstrable.
Since I also believe human consciousness, "qua human," does not really exist until it has reached the stage where it can (whether it does or not) rationally and volitionally develop cognition (verbal knowledge), it is not yet a human soul or consciousness. This does not deny consciousness; it does not deny "humaness," in the genetic sense; it does deny that in terms of moral and political principles, true humaness does not exist until conscious rational choice is possible.
We do not hold any creature incapable of making conscious choices morally responsible. Moral responsibility cannot be divorced from any correct concept of rights. Only those capable of responsibility have rights.
Hank