Parasitic conjoined twins have the same physiopathology then conjoined twins. They are joined to an otherwise normal fetus, the autosite. They usually consist of externally attached supernumerary limbs, sometimes some viscera, rarely a functional heart or brain. They are united in the same sites of the conjoined twins, ventrally, sharing the same yolk sac or dorsally sharing portions of the craniovertebral axis and often the neural tube as well. The frequency in the dorsally parasite conjoined twins is over five times greater than those united ventrally. The chromosomal pattern were generally identical.
In the present case the union is by the rump (Pygopagus). More than a half of the Pygopagus are incomplete. The parasitic twin usually has bony limbs (the most common), perhaps genitalia and a anus, and neural connections, or vestiges of any organ, like bowel, bladder, kidney, or other rudimentary organ, or simply a mass between coccyx and the anus, like a sacrococcygeal teratoma. Neural connections above the sacrum are rare. Two thirds of the cases are females.
Surgical separation was reported in literature and almost all survived. A fibrous or cartilaginous attachment to the coccyx was noted and large blood vessels too. In cases with union of neural tube, the cord could be divided and successfully separated. Some incontinence can occur.
Reference:
Rowena Spencer (Conjoined Twins: Developmental Malformations and Clinical Implications)