Dual Pistols is a Primary Power Set for Blasters and Corruptors and a Secondary Power Set for Defenders.
You're a master of dual-wielding pistols. While anyone can pick up two handguns and attempt to use them, you've trained yourself to be uncommonly effective. But even more, you've discovered or developed a way to upgrade your pistols so that they can apply various effects to the shots they produce. You can shoot ice bullets, flaming incendiary rounds, or corrosive bio-toxin projectiles.
Taking this Power Set provides proficiency with all forms of firearm pistols, both revolvers and semi-automatics. However, you almost certainly have a specific weapon that is more than a mere handgun, and even something beyond a modern military weapon. Your pistols are likely unique, and certainly special to you in some way (even if only by virtue of their ammunition-changing features). They may or may not be irreplaceable, but losing them is never inconsequential. If you lose one of your weapons, you can make do with any other pistol that you pick up off a fallen foe or otherwise acquire. However, you must complete a short rest studying any new weapon before you are able to use any round aside from standard ammunition. This time represents you constructing or adding the necessary components for those features, or unlocking and enabling them if they're already built into the weapon.
Regardless of its specific form, your pistols are ranged attack weapons that deals 2d8 piercing damage, have a default range of 60/180 feet, and a capacity of 9 rounds. If you run out of ammunition, you may use your Action to swap out both magazines (assuming you have full ones handy -- and knowing you, you do).
As with all ranged attack weapons that you are proficient with, you add your Dexterity Modifier and your Proficiency Bonus to your attack roll, and just your Dexterity Modifier to the damage roll. Normally, when dual wielding any weapons, you are not able to add your Dexterity Modifier to the damage roll of the second weapon. You are not limited in this way when using pistols.
At 1st level, you gain proficiency with all pistols. You may add one or more Endurance Die to the damage roll of any pistol shot you take or feature you use. In addition, you may swap out your magazines using your Bonus Action instead of your Action. Finally, your skills allow you to increase your effective range with all pistols to 80/240 feet.
You are not penalized for wielding a second pistol; you add your Dexterity Modifier to the damage of both guns. You still must use your Bonus Action to shoot your second weapon per normal dual-wielding rules. If you successfully hit a single creature with both pistols in the same turn, and you roll at least one Endurance Die as part of at least one of the guns' damage, your target must make a Constitution Saving Throw or be knocked prone.
Being a Pistol Expert provides additional features. You may use Suppressive Fire at 4th level if Dual Pistols is your Primary Power Set or at 8th level if it is your Secondary Power Set, Executioner's Shot at 7th level if Dual Pistols is your Primary Power Set or at 11th level if it is your Secondary Power Set, and Piercing Rounds at 10th level if Dual Pistols is your Primary Power Set or at 14th level if it is your Secondary Power Set.
In addition to being highly adept at wielding two handguns at the same time, you are also capable of repeatedly firing shots from your guns without a significant drop in accuracy. You can use Empty Clips at 1st level, Bullet Rain at 3rd level if Dual Pistols is your Primary Power Set or at 6th level if it is your Secondary Power Set, and Hail of Bullets at 12th level if Dual Pistols is your Primary Power Set or at 15th level if it is your Secondary Power Set.
You may only use these features if both weapons are at least half-loaded, and using any of them will empty both guns of bullets.
At 2nd level if Dual Pistols is your Primary Power Set or at 4th level if it is your Secondary Power Set, you've developed or discovered a way to modify your guns so that you can apply alternate damage types and secondary effects to your bullets. Using your Action, you can put either or both guns into one of three alternate modes: Cryo, Incendiary, or Chemo. You can swap each gun independently. When you hit a target while the gun is in one of these modes you gain special benefits:
You may use alternate ammunition when using one of your Pistol Expert or Quick Shooter features. You can set each gun independently for Pistol Expert, but when you use a Quick Shooter feature you must set both pistols to the same damage type.