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.
Then there’s the case of the 17th-century Italian gentleman Lazarus Colloredo, who wore a cloak to conceal the protruding upper body of his brother Joannes Baptista. The historian Henri Sauval claims to have played the twins in a game of handball in Paris and that Lazarus afterwards boasted of having killed a man without repercussions, since his brother was innocent.
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).
All of the above assumes that one twin is unambiguously guilty, and the other is unambiguously innocent. In real life, it’s hard to imagine such a clear-cut case. For example, a jury might be inclined to believe that the “good” twin acted as an accomplice, or perhaps an accessory, to the crime after the fact. This charge would apply if one sibling knew that the other had committed a crime—which seems likely under any circumstances—and that he intentionally provided assistance or comfort to his sibling rather than calling the police at the first opportunity. If the good twin were convicted of an accessory crime in federal court, he’d be subject to at most half the prison term appropriate to his evil brother. In some states, however, it’s legal to harbor a fugitive if that person happens to be your sibling.
One more way that a “good” twin might be convicted, even if he took no part in the actual committing of the crime: In some states, he might be found guilty of not stopping his brother. Although as a general rule, common-law tradition dictates that you can’t be held accountable for something you didn’t do, 10 states have so-called “duty to rescue” statutes. These require innocent bystanders to call the police or reasonably attempt to aid a victim in distress. (In four of these states, siblings of the offender are exempt from the law.) If one twin tried to stab someone, the other might be expected to grab his arm or drag both of them to the ground. The penalty for failing to rescue is usually a fine, though some jurisdictions allow for up to a year in prison.
Bonus Explainer: Could an innocent conjoined twin be compelled to testify against her evil sibling? Absolutely. In the United States, spouses tend to have special privileges when it comes to testifying against each other. A handful of states offer similar protections to parents who don’t wish to testify against their children. But siblings—conjoined or not—have no such right.