Well, if I'm with a friend while he commits murder without attempting to stop him I will most likely get some kind of punishment.
We regard a schitzophreniac as one person with a mental disorder. Even if there were two distinct personalities within one body we would still hold one accountable for the actions of the other.
Anyway, we regard conjoined twins as two separate persons who share some part of physical body. Some even share vital organs. When does it become more accurate to regard this as one creature with two heads?
In my opinion the answer to the problem explored in the article is a matter of defining precicely what conjoined twins are, legally speaking. I can't see how one half of a conjoined twin can murder someone without the other half knowing anything about it.
And besides, the question of wether or not an innocent person can be punished for a crime applies beyond this issue. If a man is sent to prison for murder, his family will suffer for it. He can no longer provide for his children or his wife.
But this we lay on the man himself. According to our thinking in this we should send the whole twin to jail and let the inevitability of punishing the innocent half be the guilty half's problem. After all, he should have thought about it before he did the crime, same way a man should have thought about his kids before doing a crime.