Hmm, that's pretty strange. Your log file looks like things are mostly working: you have two X screens that are initializing at 1680x1050, which is slightly odd since your xorg.conf file only names one screen in the ServerLayout section.
Do you intend to create two separate X screens
, or would one X screen using both your display devices
work? I've had trouble with multiple X screens on some recent desktop environments, so that might be worth a shot. I would recommend replacing the contents of /etc/X11/xorg.conf with this:
If things still don't work the way you want using that, please generate and attach a new bug report file.