I'm not sure. Wouldn't I have done that when I got the first Playstation controller working?
I ask because I don't really know. I followed a YouTube video directions to get the first dog-chewed controller working (not really knowing in detail what I was doing). One dog-chewed Playstation controller does work with RetroPie now.
I can see the other controller in the "Registered Devices" (both controllers, at the same time actually). I just can't switch between them, or better yet - get them both working at the same time.
I think the controller itself was locked up. I pressed the reset button on the bottom of the controller and it now works. In fact, they both now work at the same time. This is awesome!!