This article is related to an inter-university research project. Our starting point was exhaustive research of the international bibliography on twinship; and more specifically about monozygotic twins. About their relationship we were led to identify a passive-active couple in sadomasochistic mode. According to our hypothesis, the mother of twins becomes, in a nearly paradoxical way a “third-party symbiotic object”. Regarding this hypothesis we undertook a case analysis with a twenty-four-year-old man. We have reached the conclusion about the essential role of a primary object of investment which dives importance to the differentiation between twins.