Here is a guide on how to play Couch (local) CO-OP.
How to Local Co-op
So it seems kind of confusing starting local CO-OP in this game for some reason, so here’s a short guide for anyone else who struggled.
First you need to be in a game world, like this
Once you’re in your world and have both controllers turned on (or keyboard + controller, yada yada), you need to press a button to bring up the co-op window.
Xbox Controller: View Button See here, number 3[support.xbox.com]]
Keyboard: = key
PS4 Controller: Share (I think)
The co-op section looks like this
Once you have that up on the controller you want player 2 to use, press the relevant button again to bring up a second popup
With this popup, click the right arrow to go to the second slot, then press A (or X).
Then press A (or X) again.
Another little sackboy will appear out of nowhere.
Video guide (kind of sucks sorry)
Thanks to lyeuhm for his excellent guide, all credit to his effort. if this guide helps you, please support and rate it via Steam Community. enjoy the game.