This was initially the intent when the feature was first being implemented, but due to technical issues had to be restricted to controllers only.
While split screen co-op is only supported when using 2 controllers, it is the UI itelf which is the requirement, and 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 recognize 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.