Instructions
Walk as slow as you did in the beginning of the mission and enter the prison room with the guards.
Approach the guards slowly as soon as Caterina starts distracting them.
Select "Throwing Knives" from your weapons list.
Target the guards by pressing the left trigger. Move the joystick around until the white "target" outline encircles both guards.
Press and hold the blue "X" button if you're playing on an Xbox 360 console, or the "Square" button if you're playing on a PlayStation 3 console. Continue to hold the button until the white aiming square stabilizes and turns red.
Release the button. Enzio, the character you are playing, will throw two knives, killing both guards at the same time.