Latitude e7440, two screens and an E-Port II docking station — how to disable the "seamless dual screen mode"?

Hello,

I am trying to configure two1920x1200  screens with a Dell E7440 and E-Port Simple II docking station. One of the monitors is connected to DVI output, another monitor is connected to the DisplayPort output. The laptop runs Linux, which is probably part of a problem, but still... Here's the problem:

I do not see two screens. Instead, xrandr outputs a 'virtual' screen 3840x1200 that is a combination of my two screens. But I want two independent screens! Any idea how to have them?

~$ xrandr
Screen 0: minimum 320 x 200, current 3840 x 1200, maximum 8192 x 8192
eDP-0 connected (normal left inverted right x axis y axis)
   1920x1080      60.0 +   59.9 
   1680x1050      60.0     59.9 
   1600x1024      60.2 
   1400x1050      60.0 
   1280x1024      60.0 
   1440x900       59.9 
   1280x960       60.0 
   1360x768       59.8     60.0 
   1152x864       60.0 
   1024x768       60.0     60.0 
   960x720        60.0 
   928x696        60.1 
   896x672        60.0 
   960x600        60.0 
   960x540        60.0 
   800x600        60.0     60.3     56.2 
   840x525        60.0     59.9 
   800x512        60.2 
   700x525        60.0 
   640x512        60.0 
   720x450        59.9 
   640x480        60.0     59.9 
   680x384        59.8     60.0 
   576x432        60.1 
   512x384        60.0 
   400x300        60.3     56.3 
   320x240        60.1 
DisplayPort-0 disconnected (normal left inverted right x axis y axis)
HDMI-0 disconnected (normal left inverted right x axis y axis)
DisplayPort-1 connected 3840x1200+0+0 (normal left inverted right x axis y axis) 546mm x 352mm
   1920x1200      60.0 +
   3840x1200      60.0*
   2560x1024      60.0 
   1600x1200      60.0 
   1680x1050      60.0 
   1600x1000      60.0 
   1280x1024      75.0 
   1280x960       60.0 
   1152x864       75.0 
   1024x768       75.1     60.0 
   832x624        74.6 
   800x600        75.0     60.3 
   640x480        75.0     60.0 
   720x400        70.1 
HDMI-1 disconnected (normal left inverted right x axis y axis)