Возможно, подразумевается аномалия с 1 семенником.

Нашлось кое-что, но все в отрывках, вместе эти два компонента не встречаются. А не может быть опечатка? Вместо monorhinic - monochorionic? Тогда бы все стало на место, поскольку речь идет о сросшихся головой близнецах, где часто встречаются различные нарушения. Но и так, похоже, речь об этом.

http://www.books.md/M/dic/monorhinic.php
Single-nosed; used to characterize conjoined twins in which cephalic fusion has left only a single nose cavity evident. http://www.brainyencyclopedia.com/encyclopedia/p/pa/parasiti...
Conjoined-parasitic twins united at the head are described as craniopagus or cephalopagus. Craniopagus occipitalis is the term for fusion in the occipital region; craniopagus parietalis is when the fusion is in the parietal region; craniopagus parasiticus is term for a parasitic head attached to the head of a more fully-developed fetus or infant.
http://www.blackwell-synergy.com/links/doi/10.1046/j.1469-07...
Parasitic twins are a rare form of monochorionic, monoamniotic conjoined twins. Early prenatal ultrasound is essential for their diagnosis and assists in determining their eventual outcome; the use of three-dimensional ultrasound may enhance diagnostic capabilities in the presence of this severe condition.
Parasitic twins: asymmetrical conjoined twins where one twin is small, less formed and dependent upon the other; very rare.