Problems with dual head in X.

[NiY]

Greets! I've been having a problem running either XFree86 (4.3 or 4.4)
or X.org  (6.7 or 6.8), in a dual head configuration. I have used a
total of four different video cards in different combinations on these
varying versions of X, and the problem is always the same. I've used
FBSD 5.2.1 and FBSD 5.3, as well.
What happens: 
I get X configured fine, and am able to run X a few times, with both
video cards working fine. After a day or so, or a reboot or three, X
stops launching. I don't use X to log into, I run it from console. It
gets to a certain point (I believe it's where it loads the video card
drivers), and just hangs. I have to cold boot, I can't reboot out of
it. I can't ssh into the machine when it hangs, I can't change virtual
terminals.. can't Ctrl-C. All of the video cards are listed as being
supported  by X.org and XFree86. Also, nothing shows up in the
/var/log/Xorg.0.log file... It's just blank. Any ideas?