Two cards, two sessions

I have two video cards in one computer. I would like to run two
completely independent X sessions on each card. I am not trying to do
dualhead or xinerama; I need a GDM login on one screen and MythTV on the
other.

ISTR a HOWTO on this - it has something to do with having two xorg.conf
files, and telling X to ignore the other card.... But I can't find it.

Thanks,

--Yan
.