I believe that “Siamese” twins are joined at any part of their body, not just the head. The medical term is conjoined.
Not that it is relevant, but my mother was told when she was pregnant with me that I would have 2 heads. This was in the days before ultrasound and she suffered from fibroids. She actually had bleeding serious enough in her 6th month to require hospitalization. Anyway, suffice it to say that I was not born with two heads, I’m not a boy so I don’t wear tefillin, and I’m not a first born and did not get a double portion of the yerusha (not did my oldest sister). Just an interesting point. I didn’t realize that it was so serious as to involve discussion in the gemara though. I’m going to ask my husband.