Well, the law being what it is and believing it is the 'be all and end all' it would probably just punish them both with jail time. And the one twin would be charged as an accomplice.

They would simply look at the case and suggest that the twin that didn't commit the murder is still responsible because he/she could've prevented such a thing. Unless of course, the murderous twin knocked the other unconscious or drugged them.. then dragged their dead weight around while they committed the act.

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