Comment 7 for bug 693151

Another workaround where you don't have to rebuild the kernels and will work for different thin clients is to put the XRANDR commands in the lts.conf file to turn off the second display during login

[80:EE:73:73:42:26]
        XRANDR_COMMAND_1 = "xrandr --output HDMI2 --auto --output VGA1 --off"

Then use https://github.com/gdi2k/mac2xrandr to setup your dual monitors once logged in.