Actually I do have a spare video card, and a slot to put it in.
The video card installed is dual monitor capable. Or is just that the drivers are not generally designed to split the resource that way?
Ah, well.. this should be reasonably simple to set up in Linux, at least. Just need two "xorg.conf" files, one for each child. Each file should then list the specific video card, keyboard, and mouse assigned to it.
Then just start one copy of X as normal (uses /etc/X11/xorg.conf), and for the other copy specify the second config file as a command line argument.
Cheers