Ah, I see your point. That's an interesting wrinkle -- I be curious if there is a legal angle we are missing that would protect the confessor, or at least reduce his sentence.
The article doesn't describe the current sentence. Perhaps it's a life sentence, and the twin is hoping for a lighter one? If so, that's a pretty big gamble.
To my mind, the “identical twin” thing without fingerprints or DNA evidence could only work in a first-time “neither one of them can be proven guilty” scenario. The gambit worsens infinitely after the trial, IMO.
I’d say, put them both in prison - it’s the only way you can be sure. :0)