Maybe I'm missing something, but in the last episode didn't Dexter re-examine one of the bodies and find some inconstancies with the cuts? Like one was done really well and the other was a bit more of a hack?
Or maybe it was the crime scenes where well displayed and the bodies where not so well done, I can't quite remember and I can't watch the video as I'm at work.
Either way there was something that lead Dexter to believe that there was two people involved.
I'm not saying the Olmos being dead/not existing/etc. theory is wrong, just I thought there was evidence to suggest that two people had been involved with the cutting up of the bodies.
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