I found out how to make this mod work with split screen, it's not hard either, just adding in a single line, go to where the controls would be configured,

"/documents/Wolfire/Overgrowth/Data"

should be a file called config, at the bottom, there's 2 settings for the flying mods you're going to add one, just copy the line for activating the mod, but replace the "key" at the beginning with "gamepad" and replace the button to either A B X or Y whichever your choice, it should work in game, load up your split screen world, and press the new button, the sound effect should play and you should be able to use the triggers to fly. . .

for copy/paste : gamepad[flying_mod_toggle_flying]: b

that will set the button to B, you can change it to any one as long as you know what it's known as in the game, idk all of them but A B X and Y are pretty self explaining . . .