Ah, I just want too much . I'm a newbie into this multiple monitor settings and how it works on Linux.
The current setup works well, just like on windows or maybe even better, because I can add panels on the other monitor as well. I just wanted to know if such possibilities exists.
Basically I wanted the copy of the wallpaper with a slideshow, but two monitors behaving like they were separate next to each other and they do, only the joined slideshow part is not working.
Having a one wallpaper stretched between all screens is just another variety that I thought may be something I can set.
However, this is just a whim and a curiosity what can be done with multimonitor.
In windows there is just one desktop setup so the wallpaper is copied anyway, but windows lack of other options. In linux those options exist but the joined desktop setup is not a thing as it turns out, at least not by default. So some details are different but in overall, dual monitor in linux seems to be more versatile and practical.
For example, in windows only one panel governs all windows which isn't ideal, because if I need to un-minimze a window I need to go back to first screen. In Plasma I can set panels or docks that are reacting only to the app that is on the current monitor. The same with global menus, titles and buttons. Plasma simply offers more practical solutions. Wallpaper is just a cosmetics but still a nice thing.