Alright, I've been working on this a little bit, and I've got it a lot closer to working completely, but TBH, the plugin loses almost all of it's "plug n play" ability when you're not using a default action sequence (or ACSET: all targets).
I'm working on trying to make it properly update the animation for the offhand on the second hit, but currently damage splitting is working.
Basically (and hopefully I'll be able to change this in the future) the best case is to use the default melee action sequence parameter if you can. Failing that, the ACSET: Each Target Action Set seems to work 100% as well, showing both animations, and splitting damage correctly with no other changes to the sequence, as long as it is your default attack skill.
I am still working on getting non templated sequences to work completely. I suspect I'll have to see if I can get someone from the VS team to assist me, as I'm pretty tapped out on stuff I can fumble around blindly with.
For now, I managed to get damage splitting to work, but the animation for the offhand hit is still showing the mainhand weapon animation, and I currently suspect that is because the action sequence command to 'show attack animation' is just pulling the animation from a skill, rather than calling the function which is supposed to be used for such, a function which I updated specifically so it would update for main and offhand weapons.
In any case, to get what I have working, I just added in a few if checks, and repeated part of what would've once been called the target sequence for the offhand, and added a script at the end to delete the _numAttacks value from the subject, so as to reset the swing counter correctly after the attack.
◆Plugin Command:VisuMZ_1_BattleCore, ACSET: Setup Action Set
: :Display Action = true
: :Immortal: On = true
: :Battle Step = true
: :Wait For Movement = true
: :Cast Animation = true
: :Wait For Animation = true
◆Plugin Command:VisuMZ_1_BattleCore, MOVE: Move To Target(s)
: :Targets (Moving) = ["user"]
: :Targets (Destination) = ["current target"]
: :Target Location = front base
: :Melee Distance = 24
: :Offset Adjustment = horz
: :Offset: X = 0
: :Offset: Y = 0
: :Duration = 12
: :Face Destination? = true
: :Movement Easing = Linear
: :Movement Motion = walk
: :Wait For Movement? = true
◆If:Script:(BattleManager._subject && !BattleManager._subject._numAttacks)
◆Plugin Command:VisuMZ_1_BattleCore, MOTION: Motion Type
: :Targets = ["user"]
: :Motion Type = attack
: :Show Weapon? = true
◆Plugin Command:VisuMZ_1_BattleCore, MOTION: Wait By Motion Frame
: :Motion Frames to Wait? = 1
◆Plugin Command:VisuMZ_1_BattleCore, ANIM: Action Animation
: :Targets = ["all targets"]
: :Mirror Animation = false
: :Wait For Animation? = true
◆Plugin Command:VisuMZ_1_BattleCore, MOTION: Wait By Motion Frame
: :Motion Frames to Wait? = 2
◆Plugin Command:VisuMZ_1_BattleCore, MECH: Action Effect
: :Targets = ["current target"]
◆Plugin Command:VisuMZ_1_BattleCore, MECH: Immortal
: :Targets = ["user","all targets"]
: :Immortal = false
◆If:Script:BattleManager._subject && BattleManager._subject.isCurrentlyDualWielding()
◆Script:BattleManager._subject._numAttacks = 1
◆
:End
◆
:End
◆If:Script:(BattleManager._subject && BattleManager._subject._numAttacks == 1)
◆Plugin Command:VisuMZ_1_BattleCore, MOTION: Motion Type
: :Targets = ["user"]
: :Motion Type = attack
: :Show Weapon? = true
◆Plugin Command:VisuMZ_1_BattleCore, MOTION: Wait By Motion Frame
: :Motion Frames to Wait? = 1
◆Plugin Command:VisuMZ_1_BattleCore, ANIM: Attack Animation
: :Targets = ["all targets"]
: :Mirror Animation = false
: :Wait For Animation? = true
◆Plugin Command:VisuMZ_1_BattleCore, MOTION: Wait By Motion Frame
: :Motion Frames to Wait? = 2
◆Plugin Command:VisuMZ_1_BattleCore, MECH: Action Effect
: :Targets = ["current target"]
◆Plugin Command:VisuMZ_1_BattleCore, MECH: Immortal
: :Targets = ["user","all targets"]
: :Immortal = false
◆
:End
◆Plugin Command:VisuMZ_1_BattleCore, ACSET: Finish Action
: :Immortal: Off = true
: :Wait For New Line = true
: :Wait For Effects = true
: :Clear Battle Log = true
: :Home Reset = true
: :Wait For Movement = true
◆Script:delete BattleManager._subject._numAttacks
This... uhh, obviously isn't ideal. It still doesn't quite work right, and it's a lot more involved than I originally wanted. A side effect to that, though is that you should be able to do this to every skill, not just the attack skill, and it should work just as well.
Another thing worth noting is that Attack Times+ doesn't work at all with a custom sequence, with or without my plugin. The standard function is to simply make the attack skill hit more than once, and unless you're using the auto-sequence, or the ACSET mentioned above, it simply doesn't do that at all.
I'm going to start by submitting a bug report to the VS team to see if they can fix that at least. Not sure where it'll go from there, though.