Latest psychoanalytical research, based on the integration of the genetic-reconstructive method and the observative-developmental one, points out that twinship presents specific difficulties in the structuring of Self. The unfavourable feature in the structuring of Self in twins is the sharing of the mother from the very beginning of in-trauterine cohabitation passing to the relative deficiency of primary maternal care. The clinical cases of two adult DZ twins under psychoanalytical – psychotherapy treatment are herein reported to emphasize: the defective formation of a sense of personal identity; the high recurrence of cross-projective identifications; and, the tendency to assume complementary roles and re-create bonds and relationships similar to the ones experienced with the cotwin. Within the therapeutic relationship, the following features come to light: the unconsciously fantasized presence of the other twin; the strong feelings of self depreciation, inability and angry emptiness. These feelings seem to enhance an adhesive transference, presenting difficulties in separating, strong ambivalence and fantasies of a continuous analysis with the analyst-twin. In the cases we report, the twin pair relationship with its dynamics, bound to the incomplete or defective structuring of Self, represents the essence of the relationship between the twin and the object.