Double shoting is when you fire 2 shots almost simultaneously, like what the name suggests. It involves using the the reload key, melee key, and a mouse click. This means that it is easier to double shot with your melee key and reload keys when they are right next to each other so it will be easier to time. By the way, you can double shot as any class but if you are playing gunzerker and you entered gunzerk mode, you press mouse 1 and mouse 2 at the same time to fire both of your weapon shots. First of all, you start off with holding down the mouse1 button (you hold down the fire button the whole time), and then right after you press your reload key and right after that you press your melee key. So it goes Mouse 1, Reload Key, Melee Key. The way I set up my keys, is by having my reload key on R and my melee key on T, so I can press them down much more easily.