Typically, a double-barreled shotgun will be a breakaway, where the barrel bends forward to load the shells. The breakaway position is how the weapon is usually carried when unloaded so everyone can plainly see the firearm is not loaded. This method of display is particularly important when you are at the firing range.
The barrel configuration can be a side-by-side or an over/under. One significant advantage of a double barrel over a single barrel is separate choke settings for each barrel, for hitting targets at different ranges. Some sport skeet shooting competitions deploy targets at different ranges so the choke setting must be different for each barrel. This is also an advantage while bird hunting.
The recoil is off center as well as the shot pattern, which usually converges at about forty yards. Using a double barrel at close ranges means you can have an open choke because of the shot pattern coverage. For longer ranges, tighten the choke for optimal shot density on your target.
A double-barreled shotgun has been used for centuries for defense of home and business. The advantage for home defense is you can load one barrel with birdshot and the other with buckshot. People that lived in very rural areas even just a few decades ago sometimes kept a double barreled shotgun by the back door to deal with wolves, coyotes or to take care of the occasional snake and other four legged and two legged vermin.
Early models of double-barreled shotguns had two triggers, usually one behind the other. One finger inside the trigger guard pulled both triggers one at a time. Some tried the two-finger method discharging both barrels at once, which typically knocked the shooter out of position or on to the ground. Big game hunters used this method for large game. Using one finger made it nearly impossible to discharge both barrels at once. Newer models adapted to one trigger called a single select trigger, which meant the trigger had to be pulled once for one barrel then pulled again for the other. The second shot can be taken immediately after the first shot however. The two shots can be gotten off faster than a shotgun with a pump action because there is no cycling of rounds required.
A single barreled shotgun is the classic hunting shotgun, used for squirrel, rabbit and even deer and is an alternative to the more expensive double barrel shotgun. The weapon is easily loaded and fired and usually the barrel breaks away for loading and transport. The single barrel is not typically used in shooting competitions because of the need to fire multiple shots in a short time. The single barrel is designed for hunting where a single round fired is adequate. It can be used for bird hunting, but you are at a disadvantage where multiple shots may be required if you are duck or pheasant hunting.
Usually, farmers and others that lived in the country carried their single barrel with them to take advantage of game animals wandering nearby and for protection while out plowing or tending their crops. They carried them to the livestock barns to deal with snakes and other varmints. They usually had birdshot, slugs and buckshot with them and loaded whatever was called for at the time.