5

I've been using this feature (found in E17+) for years now, and it is extremely handy if you're a programmer dealing with multiple documents, terminals and online references spread across 2 or more displays.

In E17 there are shortcuts for:
->Switch to Desktop X
->Switch to Desktop X (all screens)
distinctively; in KDE/Plasma, only the later is available (though it is not specified like that).

So, basically, it would be handy having the Screen_X having Desktop_A active, while Screen_Y has Desktop_B active.