Sorry for the delay! You’re able to play co-op as long as you have at least one xinput controller (Xbox controllers), or use a PS4 controller with ds4windows.
To toggle co-op, you can press start on P2’s controller at any time and press right on the dpad to assign them to player 2. Alternatively, if you press start on the overworld, you’ll see an option to assign input by pressing left/right- pressing right will assign it to p2.
There’s an update on the way to allow more input devices, and some QOL improvements for adding a 2nd player