joystick is already installed, so I just ran the command, and it said ‘file not found’, which lead me to believe js0 was invalid. So, I ‘cd’ to /dev/input and found out that the pi is assigning the controllers js2 and js3, so I went into retroarch.cfg and changed the indexes to match, and low and behold, they both work!

Thanks for the tips in getting this setup!