Unlock Free Mode without having to beat the game normally, this lets you play in co-op immediately.
Free Mode Cheat Code
Left Left Left Left
Right Right Right Right
Left Right Left Right Left Right Left Right
You should see the Free Mode selection light up and the game will save. From here you can play the entirety of Free Mode either solo or in co-op. However, you will still need to individually unlock the extra characters in Story Mode.
Thanks to Virtua Sanus for his excellent guide, all credits belong to his effort. if this guide helps you, please support and rate it via Steam Community. enjoy the game.