This little device is A3 Tactical’s Double Picatinny Clamp. As the name implies, the Double Picatinny Clamp is designed to mount one Picatinny rail to another, providing, as A3 states, “virtually limitless possibilities… only limited by your imagination” in regards to accessories, guns, and gear featuring Picatinny-spec rails, at least. It’s CNC machined from 6061 aluminum, and its design prevents fore/aft movement of the rails, as well as preventing marring of your rails thanks to dowels that are machined slightly taller than the industry-standard Picatinny groove.
Of course, A3’s Double Picatinny Clamp does also give you the option to attach entire firearms to one another, creating combinations such as this:
Or this (please note that NFA regulations will apply when using this product for this purpose):