Dear @Paulus+Kieviet this is muchly helpful, I have just tried it out.
I had not mentioned but I am working with several floating windows for a long time: I have some special friends that deserve the size of one entire screen like biblical key terms, wordlist and interlinearizer. So I have those as floating and I bring them up efficiently with ALT+TAB when needed.
But for me your idea is new and is a great step towards my request: Have a second window, maximise it easily (via Windows throw-upwards feature) on a second screen, and then drag more ressources into that one. I had never tried to plopp several ressources into one of those floating windows.
My new other-main-window will be like one of those floating house-boats in Amsterdam that never really go anywhere. They are big and cosy, and have flower pots on deck and a fixed mail box on the pier.
In what I consider my “main PT window” I have a lot of stuff like probably most users. And it is this main window that I want to span over both physical screens.
Now with your input things are much better, since Windows can very easily und properly (and without hiding anything behind the taskbar) manage two full-HD-windows running for one application.
I have already done the tests with closing the program and re-launching it (simulating a fresh work day). Yes, I can confirm, that each “sub-main-window” gets placed to its respective screen, like on the previous work-session. And the ressources inside get properly arranged anyway, had no doubts about that.
I also tried dragging a window to the side and re-applying the inbuilt “layout” that I made for my test:
All the arrangements over both new “partner-main-windows” get correctly re-applied as expected. Joy. The only detail that is not yet covered is the position-on-screen of each window. But since I want “full screen coverage” it is always easy to throw any widow upwards and have it snap to full screen, if it gets ever dislocated.
Thank you for having taken the time to share. I could mark my question as solved now. But I get “greedy” and will wait a little longer if other experienced users want to add their own advice or tricks.