That's the Dual Camera System designed by Spider, a company that produces on-body photographer's gear. Two cameras too much? They've also got a Single Camera System that you can stick on an ordinary belt.
Regardless of whether you're single- or double-gunning it, the company has put some careful thought on how best to hang heavy DSLRs off the side of your body without damaging the gear.
This video provides a closer look. I like the wisdom of the two-pin system, and the way they've designed it so that you can still grab the camera from the bottom while shooting in portrait mode.
If you don't feel like committing to the entire rig, they've also designed the Black Widow, which clips to any belt:
Whether it was designed in or accidental, that audible click as it locks into place is a nice feature, and one that I wish was designed into more belt-hanging objects. I might just be a klutz, but if I'm wearing long johns in winter and try to hang something like a tape measure or tool off of a belt or a pocket, I can't feel the clip through the bulk of the clothes—so I mis-clip it and it just drops to the ground. (This most often happens when there is another person in the room evaluating my competence.) If I was a Wild West gunslinger, after shooting someone I'd go to put the gun back in the holster—then it would hit the floor and shoot me in my own ass.