How can we punish conjoined twins?

I just read this article I found via neatorama, about the possibility of punishing one half of a conjoined twin.  The set up is fairly simple, imagine a set of conjoined twins, one of them commits a crime, the other one objects and cannot stop the other twin from committing the crime.  So we have one innocent person and one guilty person.  But how can we punish one without punishing the other?

The essay takes it from a legal perspective, and it comes to the conclusion that we really can't punish the guilty party legally, and I think he might be right.  Even morally speaking, it would be hard to justify punishing an innocent person to ensure a guilty one is punished.  We built it into our laws that we'd rather have guilty people go free than innocent people go to jail.  The alternative, requiring some kind of separation of the two, would be an odd kind of punishment as well.  We usually don't force surgery onto convicted criminals, much less innocent people. 

But this suggests that conjoined twins are legally and ethically impervious to punishment.  Imagine you were the parent of a set of conjoined twins.  How would you punish them if one snuck out, to the objections of the other?  Put a bag over the head of the guilty party?  I'm really at a loss as to how to think about this particular puzzle.