Reply #36: Yes, there is another configuration. Over & Under.

[Silverhair]

On barrel atop the other. Side-by-side has a tendency to recoil slightly to the left or right depending upon which barrel is fired. That difference can make them a bit more difficult to shoot extremely well, if the shooter is good enough to feel the difference. Over & Under shotguns don't have that problem.

In really extreme examples, about a 70 years ago, there was a fashion for custom made multiple barrel configurations, having two shotgun barrels, and one to three rifles of differing calibers built in.

There are also some rifles built in the double barrel (both configurations) mode. They are usually very large caliber rifles - elephant guns. (I do not agree with the hunting of elephants at all.)