Dual Monitor mode in XFCE not working with xrandr

I am using Manjaro with XFCE on a laptop (Thinkpad x240). I have a docking station (Thinkpad Ultra Dock) with a monitor connected and that works like a charm. Today, I received my second monitor and as I plugged it in, it just mirrors my first display. There's no setting in XFCE to set it as third monitor (laptop display included).

However, in the settings editor are both display listed (first with HDMI, the second with DP) and the position is set accordingly to the information I found on Google (set the X-position of the second monitor with the resolution of the first monitor, which is 1920).

I changed this setting with xfconf-query -c displays -p /Schemes/Apply -t string -s 'Default' --create

Still no change, even after restart. I tried everything, starting with xrandr --output HDMI1 --left-of DP2 and vice versa.

I also tried arandr (xrandr GUI), and even though both monitor are set in a line in arandr, they just mirror the same desktop.

If you have any ideas how to fix it, please don't hesitate to let me know.

uname -a

Linux bx 3.16.7.3-1-MANJARO #1 SMP PREEMPT Fri Dec 19 18:34:00 UTC 2014 x86_64 GNU/Linux