elfkin477, here’s duals on a Mac. This particular Mac is a PowerBook so one of the two “monitors” is the built-in LCD screen (the one on the left). Physically they are about 7 inches apart and angled, whereas in the screen shot the look like the edges touch. The LCD is of lower resolution than the CRT monitor on the right, so there’s an empty swatch at the bottom on the screen shot.
I’ve put up the Displays Control Panel so you can see the controls for positioning the screens relative to one another (I could stack them vertically or make the one that’s physically to my left be perceived by the computer as being on the right, but of course it’s easier to mimic the physical arrangement).
This is OS X, but the same basic controls and ideas apply to MacOS 9 (and 8, and 7…Macs have actually supported as many monitors as you could afford video cards for since the Mac II back in 1987 under Macintosh System 2).
Here’s my old 7100 with three, a photograph rather than a screen shot so you can see how one would array them physically.
I’ve never used a PC with multiple monitors but I imagine the implementation and controls are similar. All the modern Microsoft releases support it, going back to Windows98. (& NT 4.0? or did the NT-kernel family not get multiple displays until Win2K?)