The reason is rather simple. You're effectively halving your numbers of men if you have a musket that's double barreled. If you're making a double barreled musket, you're making two barrels of two muskets into one, so you're now limiting the amount of soldiers on the battlefield. While you could argue a quality over quantity, within this time period, due to the inaccuracy of the weapons it isn't viable to spend unnecessarily on wonder weapons like this.

Generally, with the exception of the double barreled shotgun, most double barreled guns were for the nobility due to the expense of making them and of those weapons used, most were used for hunting.

Two barrels you have to load doesn't exactly improve your rate of fire much. In addition it's making the weapon heavier, more complex, and harder to use with a bayonet.

Weight would be a problem. Most soldiers in the 18th century carried a musket. The defining characteristic of a musket as compared to an arquebus is that it shoots bullets large and fast enough to pierce through armor. But muskets were only barely possible to carry and to shoot without support. A double-barreled musket would be impractically heavy to use for an infantry soldier.