Shortly after you start chapter one and complete some training you will be asked to make your first morality choice. Pick A to let the guy live and you will unlock The Optimist. Pick B and you will unlock The Pessimist. Right after the achievement pops you can hit start and “restart from checkpoint” to unlock the other achievement. You will start very close to this spot.
Note: Just make sure to stick with whatever decision you picked last for the rest of your morality choices throughout the game so you can unlock either Heartless or Purist.