I want to connect two external displays to my laptop running Ubuntu 18.04 LTS.
Setup: My laptop (Dell XPS 17 l702X) has an internal graphics card (Intel) and a dedicated graphics card (NVidia GeForce GT 555M). It also has DisplayPort and HDMI slot. The DisplayPort is connected to the Intel card and the HDMI port is connected to the NVidia card. I am using the proprietary NVidia drivers (nvidia-driver-390).
Problem: If I use HDMI only, I can get an output on the external monitor. Otherwise, I only get an output on the DisplayPort monitor, never on both of them. In the Display settings, only one monitor shows up.
Efforts: I also tried to switch between graphic cards using
prime-select and the "NVidia X Server Settings" tool and also the open X.Org drivers without any success.
Question: How is it possible to make both displays run under Ubuntu?