You can also modify the “Skip Assigning Gamepad To Player 1” setting at run time by changing “bOffsetPlayerGamepadIds” of the class “UGameMapsSettings” directly in your game instance.
Full answer in that post : https://forums.unrealengine.com/unreal-engine/feedback-for-epic/48403-keyboard-p1-gamepad-p2-please?p=1549367#post1549367