Change Controller ports?

Trying to work out how, if at all, I could switch controller ports in RetroArch.

For example, in Battletoads (NES) if you start the game with only Player 2 plugged into the console, you can play as the orange character instead of the green one.

Is there a way, once the game is loaded, to just change what port the controller is plugged in, so to speak?

I think you’d need to go into settings > input > user 2 input binds and set the device index to your gamepad and then remove it from user 1’s device index.

So there wouldn’t be a way to do it if, say, player 1’s controls were mapped to WASD and player 2 was mapped to IJKL on the keyboard? It would have to be using native controller mapping?