This is a big issue for most split screen games made in UE4, though I have found a fix for this game after some time looking around. In the game files there is a file called Engine.ini located in Forts\Saved\Config\WindowsNoEditor. In there you should add the following lines:
[/Script/EngineSettings.GameMapsSettings]
bOffsetPlayerGamepadIds=True
That will hopefully fix your issue and make the game playable with one keyboard and one gamepad.
Sorry for the inconvenience, hope you have fun in split-screen!