Random inappropriate legal question

Reading an article about conjoined twins this morning made me wonder something: Suppose conjoined twins never separate. How is it determined whether they are legally separate people? Is it about whether they have two functioning, separate brains?

Thinking about marriage in this context sort of makes my head spin -- I mean, with Chang and Eng, the famous "Siamese" twins, it was not that strange, because IIRC they each had separate reproductive systems. But with some others, such as the Hensel twins, reproductive systems are shared. Will those girls be able to marry? -- is there any marital structure available to them that wouldn't look like polygamy to somebody? What about if they have children? Who's the mom on the birth certificate? And wouldn't it suck if one person consented to sex the other person didn't want to have?

(If I were a conjoined twin I'd probably get really tired of everyone's questions being about sex, hence the title... but I like to think about the ways unusual circumstances can break laws, and in cases like these, family law seems very easy to break.)