still doesn't work
The attachment didn't work for me, but I found the Xinerama HOWTO with Google. It basically told me to do all the same things I have already done. I got my bus IDs from scanpci rather than X -scanpci, but that's only because X -scanpci gives no output at all (yes, I stopped off gdm first). Note that I get a different error when I have the wrong BusID.
Does anyone have an example with two identical cards?