If you have a built-in (Intel) video card, installing i810rotate
may help - it fixed the problem for me, and also allowed me to switch screens
with bash scripts.
Also, if your xorg.conf file (or XFree86 if you didn't uprade to
x.org) mentions both video devies, the Fn-F8 key won't work. Take out all
definitions for the second device (only 1 Monitor section, only 1 Screen
section, only 1 Device section). If you have both devices defined, X
doesn't appear to allow you to switch between them (appropriately IMHO, since
it's against the config).
This will cause you to get "could not find device section for
deviceID PCI 0:2:1 or something", but that shouldn't stop booting.
Rob
Disclaimer: I'm just learning.