atlantodidymus

at·lan·to·did·y·mus

(at-lan'tō-did'ē-mŭs),
Conjoined twins with two heads on one neck and a single body.
Synonym(s): atlodidymus
[atlanto- + G. didymos, twin]

atlantodidymus

(ăt-lan″tō-did′ĭ-mŭs) [ atlanto- + didymus]
Conjoined twins with one body and two heads.