Hey there.
Unfortunately, there are no plans to bring this to MZ. The MZ version of the dual wield plugin was deprecated by an update to the VS_BattleCore plugin (which it requires to work), where they essentially built in most of the battle functionality of this plugin into the battle core. As such, the MZ version basically only contains the equipment related changes, such as the ability to lock weapons to the main or off hand, make a weapon two handed, or allow a shield to be equipped by a dual wield actor.
The other issue with it, was that switchable weapons for MV requires independent items to work. Without that feature, switching one sword to two-handed mode would switch all swords to two handed mode, including those equipped to other party members, or in the inventory. While I did make a patch to get independent items to work in MZ, I never managed to find the time to get switchable weapons working with it, due to the above change in the VS_BattleCore.
There are plans to revamp the MZ dual wield plugin, to make it so it can be used without the VS BattleCore plugin, which would make it a lot more worthwhile, and when I do that, there is the chance that this plugin gets included with it. But work on that had ground to a halt for now as I deal with other things.
P.S. I see you recently bought the MZ Dual Wield plugin. I'd understand if using it for a while if you'd want a refund, since it hardly does anything close to what the MV version does for MV. In that case I might suggest getting the MV version, and using a plugin like FOSSIL to get it, and the switchable weapons, (and item core) working in MZ. Although I haven't heard of anyone trying that, the creator of fossil does have a very large compatibility list, and might be willing to work to get these working as well, if they're not already.
~Ramza