This question is an exact duplicate of:
I am using Ubuntu 14.04 kde plasma 5 - Dell xps laptop - graphics card nvidia GeForce GT 550m.
My two monitors won't work together! When I set the laptop screen as principal monitor, the external monitor is just blank. I can see the cursor, I can drag windows, but this is all I can do. And vice versa, the same thing happen when I set the external monitor as principal display!
I have tried several drivers (nouveau, proprietary...), I've tried to change settings in kde monitor settings and nvidia x server, I even tried to speak gently to my computer
Can you help me please?