Split screen co-op does indeed require two controllers. Or more specifically, it requires the controller UI. At least in Windows you can use a virtual gamepad, like
Headsoft's VJoy Virtual Joystick Driver (with the option set to block keys, in order to stop keystrokes from also being sent to the game).
After setting up the keys, you would need to use
x360ce to get the game to recognise the virtual joystick. The default settings should be fine, though opening the x360ce program can make it easier to test the key layout and assignments.
I didn't do a full gamepad configuration when I tested this with the original release of the game, just the left stick, Dpad, A/B/X/Y and start buttons, enough to see if it would work, and get local split screen going with a keyboard and Xbox 360 controller.
Keyboard + controller support was initially planned, but due to technical issues when that was being implemented, split screen co-op had to be restricted to controllers only.