[JoeRedskin]

Here are some thoughts from a lawyer who has thought about it:

As actors under American criminal law, conjoined twins present paradoxical obstacles to the application of traditional methods of criminal punishments. The Western notion of individuality precludes such duplicitous beings from orthodox measures to remedy criminal action, particularly the crime of murder. Constitutional limitations of due process and guarantees of life, liberty and property militate against equal treatment of these actors under the law. I believe that within our Constitutional framework, the only thing to be done in this situation is to release the conjoined twins. Even if the jury sentenced the conjoined twins to death, the court would have to commute the sentence and release the twins. The guarantees of due process under the Fifth and Fourteenth Amendments prohibit punishing an innocent actor. Furthermore stare decisis, the doctrine that states that courts must follow the precedent of preceding and higher courts, presents an additional danger. If the court decided to punish the innocent twin despite her innocence, there would be nothing to stop the State from punishing others who have not been convicted of committing crimes (see also enemy combatants, plea of not guilty by reason of insanity, etc.).

While this solution may seem like a grave injustice to society, consider the innocent parties injured through mistrial, the criminals released because of shortcomings of shoddy police work, and statutes of limitations preventing the delayed filing of charges despite ironclad certainty. Such is the nature of our legal system. With these limitations come the freedoms and guarantees of the Constitution, preventing an overreaching government from undue interference into the lives of private citizens and frivolous legal action.

By the way, this has actually happened in the past:

The question posed is not purely speculative. According to 18th century French historian Henri Sauval, a murder of the kind presented was perpetrated in the 17th century by Italian conjoined twins. Born in 1617 in Genoa, two boys were held together by the stomach. One twin was completely healthy while the other was mute, deaf, and blind. Sauval records that the healthy twin stabbed a man to death and was tried, convicted, and sentenced to death. However, the twin was not executed “on account of the innocence of one of its component halves.” It was impossible to put one to death without twin killing the other. Unfortunately, Sauval failed to mention whether they were subsequently incarcerated or released after the death sentence was commuted, leaving this scenario ripe for legal analysis in the abstract.

Both quotes are from: Half Guilty - Nick Kam