The second one know of the crime, so is an accessory after the fact, if not an actual accomplice for enabling the commission of the crime.
How many jets are killed by sharks?
At a minimum, extended visiting hours.
Well, this is basically the solution they came up with in Welcome to Night Vale when 4 out of 5 heads of a 5-headed dragon tried to kill the mayor.
Objection. Assumes facts not in evidence. 
No? Yeah, I felt cool for a sec. :o
So what if one twin is a drug user and the non-user tests positive because they have mixed bloodstreams.
Two threads, and nobody brought up the Tales from the Crypt episode “My Brother’s Keeper”, which deals with this exact scenario?
Long story short, two cojoined twins disagree about separation surgery. One was all for it, and the other didn’t want to take the risk. Eventually, the “for-it” twin commits a murder while drunk, and subsequently the two pass out. When they awake in the hospital, the murdering twin is jazzed about how they can’t punish him without punishing the innocent twin, so he will get away with it.
The twist, of course, is that the innocent twin, who had heretofore resisted signing the consent for surgery, had done so before passing out, and the story ends with him being wheeled out of the room while the cops arrest the other, now separated, twin.