"Twin Stab" skill:
With this skill, the powerful thief can leverage his initial damage when backstabbing the victim. If the thief lands a successful backstab he will have an opportunity to use his offhand piercing weapon to thrust it in the back of his/her victim as well. Since the Twin Stab makes use of the offhand weapon it is not as powerful as the intitial backstab.
Twin Stab Tome:
The Twin Stab tome is found in the Haunted House inside of the Midway. One must make their way into the small town inside of the Haunted House and find a boarded up window. Open this window to gain access to a small room. Search this room for a key. Once you have acquired the key you must find A Decrepit House and find a hidden exit. Make good use of your key, and enter at your own risk. On the other side you will find a chest with the Twin Stab tome inside. (A blood soaked book)
Note: The tome is a random load, sometimes you must try more than once to get one.