I think your question is somewhat incorrect...conjoined twins often develop
from a single egg which has incompletely separated, which if it had
separated would have produced a set of identical twins. Two eggs ovulated
at the same time and fertilized at the same period of a woman's cycle would
give non-identical, fraternal twins;.
Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy
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