If you want to go back to last century: I think System 4.1 (shipping with the Macintosh II in 1987) had multiple-monitor support.
Indeed. I had two monitors on my Macintosh II back then - one color monitor and a portrait paperwhite monitor (forget who made it) for page layout.
Wow, memory lane. Yes it did, with the new 'ColorManager' that could display the same window spread across the beautiful 640x480 color monitor and a B&W monitor. Very cool in the day.
I like my dual setup with one monitor in portrait for documents, it was pretty cool for the time.