Bourbon in a decorative glass decanter
Food - Drink
There's A Simple Difference Between Single And Double Barrel Bourbon
By law, bourbon must age for at least two years in new, charred oak barrels, but after it's completed its single barrel requirement, it can be aged again in a second barrel.
While the first instance of double barreling occurred accidentally, it is done intentionally these days to enhance the flavor and refine the bourbon.
Considering that around 60% of a bourbon's flavor comes from aging in the barrel, it makes sense that using a second barrel would lead to a more flavorful, complex bourbon.
There are no requirements that the second barrel has to be a new charred oak, though some distillers do use them, playing around with the char or the level of toasting.
Other distillers use port wine or sherry casks to add a different flavor profile. Aging also helps refine bourbon, so double barrel bourbon is smoother and more sophisticated.