Game director and PS4 lead system architect Mark Cerny has said Sony designed the same-screen mode to help kids play the game.
Knack is a story-driven family title Cerny has described as "kind of a mash-up between a Pixar-style animated movie and a character action game", in which mankind turns to an unlikely hero to protect them from invading goblins.
Protagonist Knack is normally only three feet tall, but he grows and transforms based on the mysterious ancient relics he picks up in levels.
In the co-op mode, player two takes control of a silver robot version of Knack, providing support to player one by fighting enemies and collecting their parts to grow bigger.
When player one is running low on health and getting smaller, player two can donate his parts to the damaged Knack. Player one will always be in charge of level progression.