I was looking at the legendary weapon effects wikia page and came to the two shot entry. It says that it "fires an additional projectile." I remembered that I recently found a two shot combat shotgun. Does its prefix mean that it fires one more pellet or twice the pellets when compared to a normal combat shotgun?
Well, let's put the speculations aside and test it experimentally!
If we compare two identical stock double-barrel shotgun (With a long barrel) in our inventory, we can see that the predicted damage for the shotgun with Two-Shot is already twice the damage of the standard shotgun:
If we test it point-blank on a gentle Deathclaw, we can observe the real damage difference between the two shots, which is just over 2x:
The slight difference can be explained by the pellets spread. I tried to reduce it to the maximum by using the Long Barrel mod and having my character placed at the exact same spot, doing a Body shot in VATS
So, in conclusion, Two-Shot will fire twice the amount of pellets from a shotgun doing correctly twice the damage.
Does its prefix mean that it fires one more pellet or twice the pellets when compared to a normal combat shotgun?
Per your link to the Wikia:
The Two shot modifier makes the weapon fire an additional projectile while still using only 1 ammo and at slightly reduced accuracy.
This means that 2 rounds will be expended. Here are some examples that will help answer your question:
If you have 10 pellets from a shotgun cartridge, 20 will be fired instead.
If you have 1 slug fired from a shotgun, 2 will be fired for 1 trigger pull.
If the two shot was on a hunting rifle, instead of 1 bullet leaving the barrel, 2 will be sent downrange.
truth is this. when a two shot is applied to the shotgun, a single bullet is added, and doubling the damage of all bullets, and screwing the spread and accuracy. it is explain why the test results were more than double damage. actually it is 2.25X damage boost if ignore other conditions like accuracy or armor.
Despite the most voted answer being that it doubles the amount of pellets, this is incorrect, and the 2-shot version doing more than double damage of the single shot actually supports the truth of how 2-shot works on shotguns and that is this:
A normal shotgun produces 8 pellets, with the damage spread among them evenly. A 2-shot fires a 9th pellet that does the same damage as the other 8. Here is some math to explain why it might do more than double damage in a testing scenario:
Firing with 8 pellets and 6 hit. Your listed damage is 80, so you do 60 damage.
Firing with 8 pellets doing 80 damage and a 9th doing 80 damage, when 7 pellets hit, including the 9th, you do 140 damage, more than twice as much as the first test.