I think this is airsoft since it was posted y a UK airsoft store called Patrol Base.
From the video the long bearded guy is using what looks like a CZ Shadow as a stock on a custom stock adapter that holds the pistol by the accessory rail.
From a tactical mall ninja aspect it looks cool but in practicality it is usless. It reminds me of those Nerf blaster stocks that act as stocks. I guess for toys it is ok but a terrible idea for a firearm. Using a pistol slide for a cheek rest? I do not see the benefit to having the pistol on the back of the Scorpion instead of in a holster. You could always pretend you are in a John Woo film and dual wield both guns. But for safety reasons, it is better to have the pistol holstered. The factory folding stock of the CZ Scorpion works very well and to replace it with a pistol seems infantile.