Were each of these tests done successively, from the same instances of the game?
Yes
Did you have one actor who had all of these skills set, and just used them one after eachother in the same battle test without resetting in between?
Yes
$gameParty.leader()._numAttacks while choosing her attack does say Undefined. Oddly enough after she attacks, and tabbing over, replacing the formula, it still says undefined. Changing back to the Skill 1 attack with no AS (Gonna use that for action sequence from now on) the number does change to 0 after she attacks.
It possibly means that the MECH: Action Effect command in the AS does not actually trigger the dual wield attack mechanic.
Dual Wield only affects the default attack skill, but it also has a function that checks if the action being used is the 'attack skill', which in RMMV was used by the YEP Weapon Unleash plugin to 'replace' the attack command with other skills. Because this plugin specifically uses the AttackTimes+ trait, which only affects the default attack skill, any skill that the engine doesn't consider the default attack will remain unaffected.
This definitely seems to be true, since my character's skill is not the default attack skill. But even so, the default attack skill with AS doesn't seem to proc correctly, hence 2 instances of off-hand damage.
It sounds like you're using an Equip Skills plugin to set the attack skill,...
Definitely. using Battle commands replacement from Visustella.
In RMMZ, there is a trait to set the 'attack skill', instead of equipping a different skill as the basic attack, try setting a trait on the class to change the attack skill to the same exact copy of the default attack skill with no action sequence, and see what happens.
After doing so, she does indeed attack twice, but both damage appear to be mainhand damage. I even set off-hand multiplier to 0.1 and she is still doing the same as her mainhand attack. This is a bit of progress though, since she does attack twice with a skill that isn't the basic attack command.
To make a skill that only strikes twice if the user is dual wielding, you'd have to add in a section in your action sequence locked behind an if check that checks to see if the actor is currently dual wielding.
Makes a lot of sense.
I could have a conditional in the AS that checks for a state given by all off-hand weapons that can trigger it (something that gives "is dual wielding" or something like that), so there's something. But how to trigger the off-hand attack eludes me.