XMONAD does it, but it's a tiling WM (Score 1)

I use the xmonad window manager, which does exactly what you want. The thing that will disqualify it in this case is that it is a tiling window manager. I use two displays. Each display is independent, but windows can be
moved between displays, or span displays. I can switch to another workspace on one display, without
affecting the other display. Any workspace can be shown on any display.

If KDE does this now, as others seem to indicate, I might consider switching to it.