As you may know there's a skill you can learn utilizing two guns and your shield. It involves shooting 1 short of your full clip and then using your shield as you switch weapons to speed up the animation so you can fire the next gun sooner...

However... if you have two identical guns you can't switch between weapons. Each weapon has to have the same level, UVs, etc and be perfect twins. Then when you use the scroll wheel or whatever you use to switch weapons you can't change, it remains on your current gun and shoots the last shot of your clip and forces you into the reload animation that this skill is supposed to avoid.