This is a controller coupler for the Atari 5200 Controllers. This binds the two controllers together to act as one to play games like Robotron 2084 and Space Dungeon as they were meant to be.
The coupler comfortably holds the controllers snuggly and safely. They stay in when you want them and when it's time to remove them they easily pop free. See the video below for a quick demonstration.
https://youtu.be/gTR9AtUGVGk
NOTE: This is for the coupler, no controllers come with it.