There have been several double barrel guns used in the United States, both smoothbored and rifled.
Until the Percussion Era, most of these were made in Europe.
These guns include the side by side guns and over/under barreled guns.
Of the over/under barreled guns some of them had barrels which could rotate about a pin at the breech. This allowed firing the top barrel, then pressing a release, rotating both barrels 180 degrees and then firing the second barrel.
This type of gun was made both in Flintlock and Percussion.
Other over/under guns barrels in the Percussion era were not rotatable, but used two hammers, one on the right, one on the left. The flame channel for one of the barrels extended down the side of the gun from the nipple to the lower barrel.
Although they existed, most of the rifled guns were not very popular in the U.S. because of the extra weight of the second barrel.
Currently, I know of only one double barrel rifle commercially made today which others have mentioned above.
In my opinion, the current lack of popularity if these double barreled rifles in the U.S. is due to their weight and the current hunting laws in many of the States.
Many States prohibit the use of muzzleloading rifles for hunting which can fire more than one shot without reloading.
Although I have heard of people carrying a double barreled rifles for hunting, they must be ready to proove to the Game Warden that one of the barrels is unloaded or be carrying the gun in one of the Centerfire Rifle hunts.
Zonie