Two persons or one?

When they go to the cinema, they pay for two tickets. Abby and Britty are conjoined twins. Or, they are one person with two heads. I strongly favor the first interpretation of this stunning feat of life. However, I do wonder how the laws will deal with them if one of them commits a crime, for example. They have a driver's license, and had to take a driving test each. How does that work in case of accidents? If they give birth, who's child is it? Can they both marry the same person? If they marry, how does love-making work? I have so many questions that I'd love to ask them...