You need two animations. The normal swing and the combo swing.
Then, if the player hits K and the sword animation is finished you set a boolean, called somthing like "combo", to true and call a timer event. This timer event represents the time the player has to hit K again to perform a combo attack. If the timer reaches 0 reset the "combo" boolean to 0 (he failed).
If the user hits K and the "combo" boolean is true, then change the animation and do the combo attack.