The skill sounds fun, but I have to ask, does the skill just apply to a single weapon or both if you're carrying two?
The answer is a players choice of none one, both or three/four with a Suspensor Harness.
Bear in mind that this comes from the House of Artifice
Rulebook and the skill description describes the
mechanics of the rule in question.
(This is the name of the skill)
If this fighter is in the starting crew, prior to the first turn, they may modify the Weapon Traits of any weapon they carry that has the Plentiful trait.
So at prior to the first turn when on the starting crew a fighter "may" modify (may gives the choice weather or not too) "any weapon". In UK English grammar, the original language in this case, the principle part of a sentence paragraph or clause comes first as you read it.
The word "may" comes before "any" in the clause so the choice of weapon is the primary here.
Given you have made a choice that applies to "any" weapon that has Plentiful the next part of the mechanics is applied. The choice "may" could be not at all or 1-4 given that the word "any" comes next. If the wording was "they may modify the
Weapon's Weapon Traits of a
Single Weapon that they carry that has the Plentiful Trait" then that would state that it applies to a single weapon.
It doesn't.
In addition it says "the Weapon Traits if any weapon that the fighter carries that has the Plentiful Trait". Traits is plural here as it then goes on to cite "Plentiful" as the single modifiable trait. As you modify one Trait with this the plural use must apply to the "any weapon" part of the clause as that is the only part of the rest of the clause that can have plurality applied to it.
The rule mechanism then goes on to detail what you replace Plentiful with and for how long including adding Rapid Fire to rapid fire weapons
"If they do so, until the end of the battle the weapon loses the Plentiful trait and gains one of the following, as chosen by the controlling player: Knockback, Pulverise, Rending or Shock. Alternatively, a Plentiful weapon with the Rapid Fire (X) trait can add one to the X value. E.g., a Rapid Fire (1) would become Rapid Fire (2).
Given the mechanics after the choice "may" are applied to individual weapons it treats each weapon picked as unique so you could apply different Traits to different weapons in the same fighter as well. A Rending Las subcarbine and a Rapid Fire 2 one can do a lot of work mowing down gangers.
So the player gets the choice to apply the skill at all to any qualified weapons the want or not and the also get the in what Traits they apply.