The description of this Trophy appears to be a bit misleading. I believe the Trophy is unlocked by killing Charlie and Fia while they are linked by Nexus, i.e., killing one automatically kills the other. I unlocked this using the machete, not a gun or bullet.
In order to earn this achievement, you first have to get to them when they are in the same place. You do this by completing the quest line "Afternoon Delight", which itself will start while completing "Space Invader", which you get automatically early in the game. Essentially, while working on "Space Invader" you need to search Charlie's room in his castle and you'll find a photo with the couple at Fristad. This leads you to search Fristad for their secret hiding place, the entrance to which is behind a complicated lock. "Afternoon Delight" is essentially the quest to figure out how to open the lock.
The full solution for that should eventually be listed under the trophy you get for completing "Afternoon Delight"
Once that is completed, you can get this achievement if you return to Fristad in the afternoon on a day where you didn't kill either one in the morning or at noon. Enter the secret area, sneak up on the pair, hit them both with Nexus, then kill one however you wish while Nexus is active, to kill both and unlock the achievement.
Again, it unlocked for me when I used the machete, not a gun.