The other solution is exact. Once you are captured, your equipment and shirts are taken and you are held face-down on the floor. Fergus and Wyatt are across from you and Deathshead makes you choose who to kill.
What's not mentioned is that at this point an autosave is triggered. Once you make your choice a small cutscene plays and then another autosave is triggered, meaning you cannot exit the game, reload, and pick the other one to earn both trophies. If you are only doing 1 playthrough, you can save yourself a lot of time replaying the whole chapter by exiting the game when you're first being held on the floor, right after the autosave, but BEFORE making a choice. Back up your save to the cloud or a USB, reload the game and make a choice. Then exit the game, copy over your save backup and then reload the game and save the other character. Now you have both trophies with just a couple extra minutes of work, instead of replaying 95% of chapter 1.