Alas, I didn't get much testing in for custom action sequences on MZ, because the thing is so different than MV.
The default attack sequence as used by the VS sample project properly repeats the target section as it's supposed to, so it might be an issue with your custom sequence, or it might be that something updated and broke compatibility. Additionally, I can only really guarantee compatibility with the Default Battle System, as the other VS battle system plugins are paid. I'm not sure if you're using one of the other ones, but that could also be an issue.
I am not familiar with this 'weapon multiplier' value. If that's something in my plugin it goes by a different name, for sure.
We can do some tests to see if the plugin is functioning correctly (aside from the action sequence thing), though. You can open the console and type in $gameParty.leader().isArmedOffhand() , and if the party leader is dual wield type, and isn't using a shield in his offhand, it should come back true. It should come back false on a non-dual wield actor, or one who is using a shield, so anyone who can't dual wield shouldn't be modified at all.
If the actor is armed in his offhand, doing $gameParty.leader().MHCalc() should return their modified main hand attack value, which, assuming the plugin parameters are still mostly defaults, is more or less going to just be their atk param from their class + their mainhand weapon + anything on their armor slots. Likewise, $gameParty.leader().OHCalc() should return the offhand attack value, which would be their base atk, modified by the offhand penalty (which IIRC is 75%) + the offhand weapon's atk value.
The next thing to check is to make sure that nothing has changed how this plugin adds the extra attack. $gameParty.leader().attackTimesAdd() should return 1 if the leader is dual wielding, and 0 if not (or more if you have traits that add extra attacks, but lets assume that isn't the case yet). If we're getting a 1 here, that means all the ducks are in a row for the extra attack to happen, and the problem is with your action sequence.
Are you trying with a specific skill, or just the attack command? Does the actor have something in place where the attack skill is replaced with something else? Does the skill being used use a custom action sequence? How is the actor given the dual wield trait? Is it on one of his weapons?
-Ramza