Re: I can't play co-op using a keyboard and steam controller.

To play both characters on just your PC (I'm on Windows 10), You must have a keyboard and a gamepad, or (I think, 2 gamepads works as well I've heard). Two keyboards won't work unless you emulate one of them to be a gamepad (from what I've heard). My solution; Download DroidJoy app on your phone (that's what I use), or whatever emulator you like, and use your phone or both your phones as gamepads. It'll ask you to go to their website, download their server and use it. (It seemed like I had to be in the game before connecting the phone as a gamepad to the server) So once in game, connect your phone as a controller, and then in the menu in the game, you should see the option to connect another controller to play multiplayer locally.
Cheers, best of luck to all!