I don't really what it is called, but it is leather, with a metal ring in the middle of the two leather pieces and two clips on the ends.
You hook the clips to your two dog's collars and your leash attaches to the metal ring.
If you need a photo, I can take one.
I had the great idea of walking PUPPY Truman with Darlene, years ago, but he grew too tall to tandem walk them.