The second twin becomes an independent person at the moment s/he differentiates from the first.
I agree. First, there’s a single celled zygote who is only one person, then it divides into more cells until the the parts of twin A separates so that half of his parts become an entirely different person twin B. So twin A became a person first, then twin B followed later.
In the case of conjoined twins, if the parts of twin A divided enough so that twin B differentiates sufficiently to form independent nervous systems, then it is only then that twin B starts to exist. There can never be a point from fertilization where no person exists because all persons were set (mechanisms and functions are already fixed in place for development) to become persons beginning from fertilization, hence marking his/her beginning.
This is somewhat parallel to organ transplant. When one’s kidney is severed and transplanted to another person, it is assumed to become a part of another person. Similarly, when parts of twin A separates during development to become independent, maybe this is when God infuses another soul to become twin B.
I thought of this because I imagined if there was a way to artificially separate a multi-celled zygote to make artificial twinning, then twinning could be a property only imposed on the outside to a growing zygote. Thus, formerly, the zygote only had the property to develop as single person (only one soul) but when artificially separated, only then did it gain capacity to become two persons (two souls). On the other hand, if twinning could only occur on growing zygotes that had the capacity for twinning from the start, then the growing zygote may have had a composite of two souls from the start, each probably governing corresponding to different divisions from the developing portions of the zygote.
This is the way I see it, but I haven’t studied philosophy so pls correct me if i’m wrong.