Comment 16 for bug 874241

Although an xrandr script could work, if the dual monitor configuration is intended to be permanent then a better solution is to supply a valid xorg.conf.

I just had to do this when switching from the nvidia proprietary drivers to nouveau (just experimenting). Jockey blows away the xorg.conf (for good reason) but no suitable dual monitor configured xorg.conf is supplied. This makes the lightdm + greeter revert to default behavior which is to match lowest resolutions on both monitors.

If you scan your /var/log/Xorg.0.log, right at the beginning you'll get the same log as the original bug reporter:

[ 8.567] Markers: (--) probed, (**) from config file, (==) default setting,
 (++) from command line, (!!) notice, (II) informational,
 (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
[ 8.567] (==) Log file: "/var/log/Xorg.0.log", Time: Thu Oct 13 14:22:37 2011
[ 8.579] (==) Using system config directory "/usr/share/X11/xorg.conf.d"
[ 8.579] (==) No Layout section. Using the first Screen section.
[ 8.579] (==) No screen section available. Using defaults.
[ 8.579] (**) |-->Screen "Default Screen Section" (0)
[ 8.579] (**) | |-->Monitor "<default monitor>"
[ 8.579] (==) No monitor specified for screen "Default Screen Section".
 Using a default monitor configuration.

I do not recommend editing your xorg.conf unless you know what your doing and can recover if the X server won't start.

The key requirements in a dual-monitor xorg.conf are:
1) a "ServerLayout" section that identifies the screen
2) the desired virtual size in Section "Screen"/Subsection "Display"
3) indicating which monitors are connected to which outputs

I've attached my xorg.conf *which is specific to my setup*. Do NOT copy it your setup! I have provided here only to aid in understanding, in conjunction with the xorg.conf man page and looking at your own /var/log/Xorg.0.log