I have a 34" ultrawide monitor, so something like this would be fine in terms of taking up real estate (for me!). I’m not sure it’s something I would use that often if it were just for working on a couple of modules at a time. I rarely do that, and I’m usually fine with the “auto-collapse” functionality enabled in preferences.
But where I can see it as being useful for my image editing is if this extra panel could include the mask manager. Currently I really don’t like switching to the other side of the window to mess with masks. Having all masking tools close to each other would be nice.
And here’s another thought: I currently rarely use the QAP because I spend most of my time in my “favourites” panel, where I have all my most used modules. But I might be tempted to use the QAP if it could be displayed side-by-side with my main panel. The QAP could have my most commonly used sliders, and then I still have my main panel open for more advanced work.
So, in summary, I think this idea has merit as a way for users to customize the interface more. In fact, the more I think about it, the more I like it. If I could send whichever modules or panels I like to it, it could be a really nice customization option. There could even be a setting in preferences for you to choose the traditional 2-panel layout or a 3-panel layout (3rd panel can always be hidden/unhidden).
The key is that this has to be purely optional (not everyone has an ultrawide monitor), and I think it needs to be more versatile than just for working on a couple of modules at a time.