A chimera arises probably at foundation: An egg, split in two cells inside one zona pellucida (probably by non-expulsion of a too big polar body) is fertilized by two different sperms. Hence, the same individual (due to the plastic bag of the zona pellucida) is carrying two different genetic make-up. But they start the life together at the three cell stages: Hence the result can only be one person. There is no theological contradiction in the fact that “a person can have two natures”.

and

“Hermaphrodite formation, on the contrary occurs at conception (9) (10). A female pronucleus is fertilized by one spermatozoa while the second polar body (of the same ovum) is fertilized by another giving rise to one human being contained in one zona pellucida. Other chromosomal anomalies secondary to anomalies at conception are known but since they occur at the moment of conception they cannot be raised as objections to conception being the beginning of a human being. The fertilization of one ovum and of its polaire body (part of that one ovum) by two spermatozoas and when the process occurs within one zona pellucida, can by the nature of the process only give rise to one human being. It is therefore completely distinct from the notion of fusion of two human beings with annihilation of one of them.

“The case described by de la Chapelle et al (11) is totally conform to such a process (in this case fertilization of a female pronucleus and of its first polar body could explain the biological observations). No case of spontaneous fusion of two embryos into one individual has ever been demonstrated in man.”

for the full paper including footnotes 9,10, and 11, see

http://www.fondationlejeune.org/Content/Chercher/Documentation/article.asp?filename=fjl431.xml

Thanks!