Law student Nick Kam has written a paper exploring a hypothetical legal scenario: from a set of conjoined twins, one commits a murder. Since justly punishing one requires unjustly punishing the other, would the guilty party escape punishment? In "Half Guilty", Kam writes:
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To consider more extreme approaches to punishing the guilty twin, the Court could order the twins separated so that the guilty twin may be punished. Even if this Solomonic option were possible in this case, as physiologically it appears impossible, this action raises grave Constitutional concerns. The Supreme Court has held that the body to be inviolate, providing slim exceptions to this rule as in the testing blood alcohol content, chemical castration, and the death penalty. This punishment smacks of the Sharia law practice of chopping off a convicted thief’s hand. Furthermore, it is hard to argue that separation would only punish one of the twins as each would be left immobile, one half of a complete body. Separation surgeries have some success as in the case of Jodie and Mary Attard (although this surgery was undertaken knowing full well that it would and did kill the weaker twin). Modern scholars estimate the rate of successful separation surgery at around 5% (see also the Bijani twins). With such dismal rates, sentencing conjoined twins to separation surgery would be the equivalent of a death sentence.
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I suppose the worst case is that the evil conjoined twin is a Jeffery-Dahmer-level sociopath who is clearly a danger to other innocent people. Punishment aside, the immediate priority is preventing the twin from hurting more people. It's not too different from a killer using an innocent hostage as a human shield. You don't want the hostage to die if at all possible, but sometimes one death is the least bad alternative. And it's killer's actions that put the innocent person in harm's way, not the law.