I was using vtwm on UNIX in 1990. Both CDE and OS/2 Warp had it from 1994.
Vtwm was interesting, because rather than separate 'desktops', what it gave you was a scrollable/snappable window over a much larger desktop than the size of the screen. This meant that you could have a huge window that you could move the visible screen over. Coupled with hotkeys to control the window manager rather than on-screen buttons, it made a very usable and flexible environment. I did find the source for it a while ago, and compiled it up again, but I'm afraid that I'm now corrupted by the need to support freedesktop extensions from more modern window managers.
And IIRC, the AT&T 5620 Blit had some rudimentary multi-view extensions to Layers in the mid '80s.
I can't remember whether the Sun 3 that I played with in the early/mid 80's had a virtual extension to SunView. I think that they preferred icon boxes to contain multiple minimised windows that you could open and close as a group.