I found a good spot for this in Chapter 2 of Nightfall. Once you reach the exterior part of the final area, you will be tasked with finding the key to a locked door in the training area. As you can see in the picture below, there’s a guard tower with 2 archers who are stationary.
Select Twin Shadow skill, then mark one archer by pressing
when your target reticle is over him. Now move behind the other archer, press
to assassinate him while pressing and holding
to allow your partner to do the same. Achievement unlocked.