help with dual monitor configuration (Ubuntu 16.04)

in system settings -> monitor I only have 1 monitor shown (build-in). I have 2 external monitors connected, this is my current monitors.xml:

<monitors version="1">
  <configuration>
      <clone>no</clone>
      <output name="HDMI-1">
          <vendor>ACI</vendor>
          <product>0x22c1</product>
          <serial>0x01010101</serial>
          <width>1920</width>
          <height>1080</height>
          <rate>60</rate>
          <x>1920</x>
          <y>0</y>
          <rotation>normal</rotation>
          <reflect_x>no</reflect_x>
          <reflect_y>no</reflect_y>
          <primary>yes</primary>
      </output>
      <output name="HDMI-2">
      </output>
      <output name="DP-1">
          <vendor>SAM</vendor>
          <product>0x0b3e</product>
          <serial>0x30333635</serial>
          <width>1920</width>
          <height>1080</height>
          <rate>60</rate>
          <x>0</x>
          <y>0</y>
          <rotation>normal</rotation>
          <reflect_x>no</reflect_x>
          <reflect_y>no</reflect_y>
          <primary>no</primary>
      </output>
  </configuration>
</monitors>

I get the following error message on xrandr:

xrandr: Failed to get size of gamma for output default
Screen 0: minimum 1920 x 1080, current 1920 x 1080, maximum 1920 x 1080
default connected primary 1920x1080+0+0 0mm x 0mm
   1920x1080     77.00* 
  1920_1080_60 (0x2b5) 173.000MHz -HSync +VSync
        h: width  1920 start 2048 end 2248 total 2576 skew    0 clock  67.16KHz
        v: height 1080 start 1083 end 1088 total 1120           clock  59.96Hz

Note that 77 Hz is not officially supported by the manufacturer of any of my monitors.

The monitors are both working at 1080p_60, but the desktop is mirrored. I want to change it to expand the desktop area.

Also, can I delete the

  <output name="HDMI-2">
  </output>

part? My monitor connection is named DP-1, shouldn't it be VGA-1, instead? Don't have DP and physical connection is VGA cable. I don't have a xorg.conf file, I deleted monitors.xml and rebooted -> the 77 Hz are still there.

Thank you!