Double Assassination takes out a pair of unaware guards in one silent move. It costs one point once Sequence 1 is complete.
Double Assassination sits at position 9 of 19 in the Stealth tree. Unlocking Double Assassination takes 1 skill point. You need to complete Sequence 1 before you can buy it. Jacob and Evie each have their own skill tree, so you buy Double Assassination once for each twin.
Air Assassination
Drop onto a target from above and kill him in a high-profile strike.
Assassin Steps
Greatly reduces fall damage, silences your landings and makes your movement much quieter.
Chameleon
In Sneak Mode, Evie becomes invisible while standing still; only enemies very close by can spot her.
First Strike I
Hitting an enemy just before combat begins deals heavy damage.