Hm, if even an old hand like Maximuuus didn't know about that Dual-Wield toggle, I guess it contributes to showing that the current combat UI isn't top notch. I mean, I don't think I've heard this problem too many times, especially compared to the number of people who don't find how to use the Druid's
Move Moonbeam or
Hurl Flame (because these add icons which can easily land on hotbar page 2, and not everyone notices that there's more than one page of hotbar). I hope Larian records this for when they design (/redesign/overhaul) the UI.
As noted in the opening post, when Dual-Wield is activated, every Attack we make will automatically consume our Action and Bonus Action, to perform the 2 attacks. Even if the first attack kills the target, in which case our Bonus Action gets wasted for a slash in the air.
Thus, at the moment (because I doubt this is Working As Intended), you have to choose between quality-of-life and tactical efficiency.
The only good thing is that the Dual-Wield toggle has a hotkey (X, by default), so you only have to think about activating/de-activating it and let your left fingers do the thing, without having to worry about spotting and clicking on a small toggle.