How would the law punish Siamese twins if one of the twins committed murder without the other being involved?
The answer: No one knows.
There isn't much case law to work with on this question, since in the United States, at least, conjoined twins represent something like 0.0005 percent of all live births—with an even smaller number surviving into adulthood. The conjoined twins who aren't separated at birth and do manage to grow up have so far tended to be more or less exemplary citizens.
That said, there have been a few recorded instances of conjoined criminality. By one account, the original Siamese twins, Chang and Eng Bunker, were arrested over a scuffle with a doctor who tried to examine them, but never prosecuted. Nor were they ever charged with bigamy, despite having taken two wives…
A more recent case concerned the Filipino twins Lucio and Simplicio Godina, who as children were rescued from a Brooklyn, N.Y., sideshow, and later toured the United States as the "only pair of male Siamese twins on Earth." In March 1925, newspapers claimed that Lucio had learned how to drive a car and been arrested after his vehicle grazed a carabao cart on a street in Manila. Simplicio supposedly appealed the case on the grounds that as an innocent man, he could not legally be incarcerated—and the judge let both twins go free. A similar story was reported in the New York Times in 1929: This time, Lucio was able to escape punishment for making an improper left turn in downtown Los Angeles.
These stories—true or not—reveal how difficult it would be to punish a conjoined criminal with an innocent twin. Let's say you wanted to throw the evil sibling in jail. There's no way to do that without incarcerating the good one as well (unless you convinced him to take a job as a prison guard). Is there any way around this problem?