During the Civil War it was not unusual for cavalry troops to carry 2 or more revolvers, which allowed a greater volume of fire between the tedious reloading process for cap & ball revolvers. I suspect that such practices continued after the war when large numbers of people were heading west to the newly opened territories, where hostile tribes and outlaws preyed upon travelers without much hope for either law enforcement or military help.
How common the practice may have been in established communities, particularly after cartridge guns became the norm, is anybody's guess. Hide-out pistols were probably fairly common with certain elements of the population. A back-up gun may have been something that was utilized by those anticipating violent encounters.
The buscadero-style holster rigs are purely a Hollywood invention. Low slung holsters and tied-down rigs are also myths perpetuated by decades of movies and TV shows. Handguns in general were not nearly as common as portrayed in popular entertainment. Rifles and shotguns were far more practical for most people.
Shooting from horseback is also something seen in many movies and TV shows. Ask anyone with experience riding horses how well that might work out in the real world. It is probably possible to train a horse to accept sudden loud gunfire in close proximity to sensitive ears, but that in itself would be a major undertaking, highly unlikely for any but a very few.
The popularized gun fights with two or more people facing off in the street and relying on a fast draw are also fictional. Those walking into dangerous situations would have their weapons in hand and ready to fire, and the rifle or shotgun would have been chosen in any situation that allowed for such choices.
