The secondary monitor support is not enough. I really want to keep the library grid view in one window, with an editable Loupe view in another window on another monitor. When I'm in Develop, I can make edits in that Loupe window.
There are various combinations possible with the current two-window setup, but none will allow two different modules to be open at the same time. You could have the Develop module on the primary monitor and a Grid view of the currently selected source on the secondary monitor, but I'm guessing what you're really wanting is the full Develop Module on one monitor and the full Library Module on the other? That isn't possible, and I guess it's not "a thing" because it's probably rarely been asked for (I've been doing Lightroom support for more than 10 years and I don't recall it ever being mentioned).
If that is indeed what you're asking for, perhaps one of the moderators can turn this thread into a feature request.
The option to add keywords while you are in the develop module certainly has been suggested before, but of course that would not necessarily require a second monitor. On the contrary, it would be nicer if you could do that on a single monitor system as well.