The best place I found for this is at the start of the second level, in the mental hospital. After the initial cutscenes/fight, you start with a gun and facing the double doors.
Open them and there is a German with his back to you. Walk directly forward and there is a second knife on the floor. Pick it up with X then dual wield the knives and push right thumbstick to throw it in the Germans back.
Just reload checkpoint 4 times and job is done!!