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Well, this idea actually happened accidentally on my KDE desktop because of a bug of the paste widget and actually this was very pleasant and fortunate!

Somehow, one of my KDE virtual desktops turned to a Gnome one, showing my gnome wallpaper and all the nautilus services enabled when clicking the right button of mouse.
All the other virtual desktops remained KDE ones and when I click the right click there, I have the KDE services.

So, I think that this is perfect and certainly suitable for many persons that want to combine the KDE services and functionalities with the Gnome services and functionalities.

Since this idea actually happened in my desktop, it would not be so difficult to be implemented as a feature.
I suggest to be an option in the wallpaper windows option.
(where you can choose "gnome environment" alongside with the globe or the slideshow etc)
Of course, for this option to be enabled, gnome has to be installed but nevertheless most people have both.

I haven't got GNOME installed right now so I can't do tests to try and replicate your scenario but you can easily run more than one DE/WM by starting X on another display.

Eg: when calling `startx` runs exec startkde, run `startx`. This'll be on ctrl-alt-f7.

Then (perhaps in your .xinitrc, depending on your distro), change to exec gnome-session, and then run `startx -- :1`. This'll be on ctrl-alt-f8.

Just use the shortcuts to switch between them.

Awesome, eh?

I think all that happened is that nautilus got launched, which airc by default starts a full-screen desktop window (which is used by gnome for its desktop).
You can get the reverse by running plasma-desktop in gnome (or other environment).
Another note: you can do alt+left-click-and-drag to move the gnome/nautilus desktop window in KDE or the plasma-desktop window in gnome; not that that buys you anything, but you can .

"gnome has to be installed but nevertheless most people have both" <-- No

Marked as pre-existing as there are several ways to achieve your idea already and due to inactivity on the OP.