This achievement is gained on the Pillow Fort Level in Rose's Room Chapter (3rd Chapter).
First, make your way through the level until you get to the flashlight that you can interact with. There will be a staircase to the right you can go up, jump across, and down into a room with two toy characters that look eerily familiar...
Interact with them simultaneously (slam the buttons at the same time) for some banter and your achievement.