Question about Targus USB-C Dual 4K

I have a laptop, the Targus USB-C 4K (USB C) docking station and two monitors which I have connected to the docking station via HDMI. The docking station is connected to the laptop via USB-C. Now I only want to work on the monitors (left monitor 1 and right monitor 2, laptop closed). Unfortunately, only the mirror mode works (same image on the left and right). Is there a way to set this or does the docking station only recognise one monitor per se?