This achievement can be easily obtained by equiping a longblade and a shortblade in your primary and secondary slots, the easiest method of completing this goal is to find two enemies that are very close together. When guards are running errands they can typically be found moving in groups of two (mostly on bigger ships), often guards will stand very close together while doing these errands. To complete the achievement, kill the first guard with the longsword, now the second guard should be in range of your shortblade, quickly use your shortblade to kill him. The kills must be within 1 second of each other. Achievement Unlocked!