I am also using the same setup, Workaround for this is to disable "mode-line" first and then use xrandr to setup dual monitor.
In my case I use like this.
gnmk-Debian~ % xrandr
Screen 0: minimum 320 x 200, current 2340 x 1600, maximum 8192 x 8192
LVDS1 connected 1440x900+0+0 (normal left inverted right x axis y axis) 303mm x 190mm
1440x900 60.0*+ 59.9 50.0
1360x768 59.8 60.0
1152x864 60.0
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
VGA1 connected 900x1600+1440+0 right (normal left inverted right x axis y axis) 443mm x 249mm
1600x900 59.9*+
1280x1024 60.0
1280x960 60.0
1280x800 59.8
1152x864 75.0
1280x720 60.0
1024x768 75.1 70.1 60.0
832x624 74.6
800x600 72.2 75.0 60.3 56.2
640x480 72.8 75.0 66.7 60.0
720x400 70.1
DP1 disconnected (normal left inverted right x axis y axis)
it works for me.