I had a dual head monitor system back when I was using Win98se. I had two display adapters (one agp and one ancient pci card) in my PC and two monitors (19" + 14" lol) connected.
It worked otherwise nicely exept the pci card had no 3D acceleration thus rendering all 3D stuff slow. Now I can't use that system anymore since there is no driver for the pci adapter for windows 2000
Nowadays there are several cards which supports directly two monitors. I would love to get one of those some day.
The dual head system just enlarges the desktop area. Say if you have your second monitor in left side of your primary monitor, you could just drag the mouse cursor through the left edge of your prim monitor to the secondary monitor...