I'm surprised no one has responded since this is an interesting purely philosophical question. The easy answer is to separate the twins if it can be done without unduly endangering either twin. I do think the suspect twin should be charged and tried for the crime of murder. The accused must have her day in court.
Without this option, some sanction must be applied to the guilty twin that would have minimal impact on the innocent twin. In medieval times they might have blinded the guilty twin. Since we are not barbaric, we have to think of something more humane but appropriate for murder. All I can think of is denying the guilty twin privileges that could be enforced perhaps by a full time guard. Prison type food, no TV, no PDAs, Ipods, required to listen to the victim's families and friends over a head set whenever they felt like calling her, being required to look at the victim's picture frequently and learn all about the victim's life assuming the victim was a sympathetic figure.
A full time guard would be expensive, but could also help the innocent twin when needed. (Privacy would be allowed to the extent possible). Prison is expensive too and such a case would be rare if not unique.
EDIT: Obviously financial sanctions are also available if the convicted twin has resources, but they are not enough.
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