I am busy with relocation currently so I don’t have time to look into your issue very soon.
I have tried to use two gamepads for player 1 and 2 in my personal project of fighting game. It worked and I didn’t have to do something special. UE4 would use keyboard and first gamepad for player 1, and second gamepad for player 2.
What I would suggest you is make sure you can use two gamepads for player 1 and 2, respectively, without my plug-in. And then try to find a way to use first gamepad for player 2 instead of player 1, without the plug-in. If you can do that, finally try to apply the method with the plug-in.