Here is the scenario: The regiments A and B are marching against each other in line formation. They are equal in number, training and moral. Every soldier of A carries - let's say - four wheellock / flintlock / caplock muskets: One in his hand and three in a bag on his back. They're all loaded. The soldiers of B are equipped normally with one musket per solider. The regiments fire at the same time and suffer equal losses after the first volley. B starts reloading, but A puts the muskets down and draws the next one. And shoots, and shoots, and shoots. So this is the situation afterwards: B fired one volley and is now ready to fire the second, but A already fired four. So B already suffered four times as many losses and their second volley will be much weaker than A's next one. A will suffer less than B with every volley until B is dead or runs away.
Wouldn't that be an enormous advantage? I can think of two disadvantages, though:
That's it. Why was this never done? Or did someone tried it? Is it a stupid idea? (It probably is.)
You're massively ignoring the advantage of formation firing. Why would I bother to equip my troops with 4 muskets when it costs four times as much to now field that unit and it slows their marching (as wikipedia says the Brown Bess, the standard musket for the British army in the late 18th century weighed about 10 pounds or 5 kgs. Why would you send someone into combat with 30 .lbs of guns when they could just carry more powder and shot?) when I can just utilize line fire to have soldiers consistently firing. The Dutch during the 80 years war developed volley fire or counter-march to deal with issues of reloading. This required a lot of training and well discplined troops, but it was quite effective. This worked by having your infantry line up in columns having the first rank fire, the second rank advances and fires and so on; it was a way to always have men shooting and pressing forward. Also, it's worth mentioning that if you equipped a soldier with 4 muskets you're making a lot of assumptions to conclude that he could shoot faster than a unit using line fire or even someone reloading a musket. Your soldier would have to grab his other musket, pull it out, make sure it was still primed (which a flintlock would almost always have an empty pan in this situation and then return to a firing position and fire. The logistics of trying to fight like that simply don't make any logical sense, it would be costly, slow down armies and probably not be anymore effective (skilled troops could probably reload almost as quickly under fire than someone trying to ready a new musket to fire even if it was loaded). Ultimately you're trying to reinvent the wheel by making it heavier and more expensive (with no way to reduce these factors) rather than improving its design to suit your needs.
Sources: Tonio Andrade, “An Accelerating divergence? The Revisionist model of world of history and the Question of Eurasian Military Parity Data from East Asia,”Canadian Journal of Sociology: cahiers canadiens de Sociologie. 36, 2 (2011): 185-208:
While we wait for an expert in Early Modern warfare to chime in (paging /u/elos_) I will offer a few general comments on what you have outlined.
Firstly, it really is impossible to reduce military engagements, in any period, to a series of abstract scenarios. The "typical battle" is an artificial construct, no two field engagements are ever exactly similar. The composition of forces, the terrain, the weather, supplies, and the character and actions of commanders, officers, and individual soldiers all play a critical role in determining the course of an engagement. No two forces are ever exactly the same in training and morale; indeed, it is problematic, if not impossible, to construct an objective metric by which such concepts (especially rather subjective ones, like morale) can be measured. Real warfare isn't a video game, in which unit "stats" can be easily measured and compared.
Essentially, no, carrying multiple muskets would not have been much of an advantage. While muskets are not a heavy weapon, they are not light either. Any additional weight soldiers carry reduces the speed and distance they can march, as well as increases the potential for exhaustion. Do remember that while on campaign, soldiers of this period already carried many pounds worth of equipment. Marching around in a wool uniform with a pack full of supplies in the sun is hot work enough. Add the additional weight of three muskets, and soldiers will likely begin to suffer.
During the Early Modern period, a trained infantryman was capable of firing roughly two to four rounds per minute. With this rate of fire in mind, it is difficult to conceive how carrying additional muskets provides a marked improvement in rate of fire. Remember also that armies did not mindlessly blast away at each other until one side was decimated. Rather, the goal was quick, concentrated volleys, then rapid advance with the bayonet. The speed of the volleys and the shock of a bayonet charge was intended to cause the enemy to break formation and flee, to be then harassed by cavalry. Any possible advantage gained from multiple muskets really doesn't mean much when the overall role of musketry is considered, especially when trained soldiers would have had little difficulty managing several shots per minute.
As for whether something like this was ever attempted, the answer is yes. Early gunpowder cavalry like reiters frequently carried 2 or more pistols which could be fired in quick succession before charging in with a sword. Many pirates in the 17th and 18th centuries were also known for carrying up to 4 pistols at once when boarding an enemy ship.
In addition to carrying multiple weapons there was also a great deal of experimentation with multi-barreled volley guns almost since the invention of firearms. Most were too heavy to be carried and had to be used from carriages like artillery, but some like the British nock gun were made to be fired as hand-held weapons.
They still never really caught on however. Loading 4 barrels and then firing 4 barrels was still never going to be as quick as load, fire, load, fire. And the amount of damage done by a single extra infantry volley at range was rarely enough to justify the expense of affording every soldier a second firearm. In addition, if a simple overwhelming volume of fire ever was needed at close ranges it would have been much easier to simply load cannons with grapeshot or load single-barreled, large-caliber muskets with hail shot than it would have been to mess around with heavy, complicated volley guns.