I've same problem and try to solve it myself for a day.
I am not good at English but just want to share my own solution that works for my game controller.
-First you must know your second controller's name other than keyboard > Go to control panel > Look for Hardware or Game controller depend on your system and copy it's name (suppose it is xxx).
-Go to Game directory that you've installed it. > Goto "actionmap" folder >open file "xinput_pad.xml" with notepad
-Look for the lines (I think at first top 5) with deviceName= then rename(or paste) in quotation mark after = to your controller's name correctly (example: DeviceName="xxx") > Then save
-Now Run game > Option > Control> Press for changing preset and select your new controller's name and assign for it (this is for player 1 now, then keyboard is automatic for player 2).
-Back to Multiplayer > splitscreen >Press Enter on keyboard (for player2).
Hope this will help you.