Double-gun problem and my solution:
In my opinion, the usage of 2 guns to quickly fire one after the other is only an issue at long range due to unrealistic and game-breaking carrying of accuracy from the first gun to the second. Allow me to explain what I mean:
Firing a blunderbuss at close range, followed by a pistol or a sniper isn't disturbing to me. It is a small compensation for the lack of numerous abilities that the player is missing out by not carrying a cutlass and it is also a high risk move because if you miss one of the shots you are vulnerable, in need of reloading two guns while your enemy is close to you and will likely slice you with a cutlass.
The disturbing problem is when someone aims a sniper at the enemy at long range, shoots it, continues to hold the aim key/button, immediately switches to pistol and shoots it too while the aiming of the sniper was carried on from the sniper to the pistol. This has 2 problems: 1) it is unrealistic to have the second weapon already aimed when you only aimed with the first one, and 2) it is game-breaking because it allows you to do something equivalent to a LONG-DISTANCE one-shot.
Notice that a short-distance one-shot isn't game-breaking because there is high reward but also high risk (performing it takes some effort and the short distance means you will likely die if you fail). But a long-distance one-shot is high reward and LOW RISK. Because you are far away so you have nothing to lose in trying (you will likely be able to run/hide if you fail).
So to solve the problem, I DON'T think some cool-down time should be imposed between firing the first gun and the second. That would be a huge nerf to guns in general and I believe the sword is already way too powerful (proof is that it is the number 1 most used weapon). I think the solution is to simply NOT allow the aiming to carry from the first gun to the second. So if I aim a sniper and then switch to pistol, I will be able to shoot the pistol quickly without aiming but with low accuracy so the second shot will likely fail at long range. This way a player can still quickly shoot 2 guns but it will only be effective in close quarters (which is ok because it is realistic and not game-breaking, for the reasons i mentioned before).