You can get both this achievement and
If you don't kill Cohen when you meet him, when he walks down the stairs in Fort Frolic, you can find and enter his appartment in Olympus Heights (bottom floor of the Mercury Suites), he will enter the room via a previously inaccessible door. You can then kill him, take a photo of his body and enter his room (the door he came in through). Gaining both of these achievements.