Damnation
38 Achievements
1,000
25-30h
People Skills
Completed the game in Co-op on any difficulty.
50
How to unlock the People Skills achievement in Damnation - Definitive Guide
Also, cheat codes still work, at least in split screen, but they will only apply to player 1. Just remember to go to the options screen and turn them on before starting a new campaign.
Since the online servers are now closed, you are going to have to do this splitscreen which means you will need a second PS3 controller. This does not necessitate that there needs to be a second person join you to actually play as the second character, you can simply set the controller down and carry on. The second person will not time out because they are idle and are not required to progress to checkpoints with you, they are literally going to stand right where you first spawn whilst you progress as the first player to the end of the level.
When you complete the first chapter in Boomtown you will unlock the trophy Double Time and you will also have unlocked the Spirit Vision ability. This makes things super easy because if you die as your main player, then pick up the idle controller and hold down to enter spirit vision, find wherever you are and face your downed main player then hit to heal them completely, no matter how far away they are. This is truly game breaking and considering you must complete the game on Insane Difficulty, it's prudent to combine these two requirements to earn both trophies in one swoop as recommended in the Road Map.
One other thing to mention in passing is that all trophies must be earned by the Main Player, meaning you must collect all emblems, and carry out all action specific requirements for all other trophies with the main player.
Simply play through the entire game in co-op mode. Co-op is substantially easier than playing solo anyway, so it's far faster to play with a friend than play alone.