That's not my argument at all. I'm fact, I never said that fetuses aren't people, or that this is true because they can't survive in their own in the entirety of the post. What I did say is that, under constitutional law, the Supreme Court ruled that unborn children are not people in the eyes of the fourteenth amendment. So this argument is seemingly irrelevant. It's also logically fallacious by getting to test the aggregate meaning of a theory by applying an extreme. For example, does the existence of a black hole causing issues within the theory of relativity mean that the entirety of Einstein's work is invalid? Of course not.

And to address the initial question, yes, conjoined twins are human. They can survive outside the womb, even though on extreme cases one cannot survive without the other. The question becomes, then, what makes a human a human? And that is honestly a difficult question and I don't even know where to begin answering that. Humanity has been attempting to for generations. We've yet to find an answer, and it's doubtful we will soon. It seems silly to base an argument over what we can't define, which is why I did my best to avoid that in my post.