Hey! Currently mouse and keyboard isn't supported in couch mode BUT there's a workaround to do what you want. If you go to Online using the keyboard or mouse and then hit A on the controller, it will add a second player. Then if you do Party Only you can play 1v1 locally like that. However, you'll probably have more fun if you do Quick Play with the two of you since there will be more players. Hope that helps!
Brett