Nvidia X Server Settings only shows 1 screen (laptop + external monitor connected)

I ran into basically this same issue, it turns out in my case (maybe yours as well), that this ends up being somewhat of a hardware issue with the shared graphics cards in laptops, where the intel (power saving) card is connected to the HDMI port and the nvidia (performance) card is not. The nvidia driver will then not work in a dual monitor setup since it defaults to the nvidia (performance) card. I don't get the design decision behind that (cost maybe?).

I found that the solution in my case was to either switch the default mode to power saving within the nvidia driver settings, or to switch to the nouveau driver (which seems to default to the power saving mode anyway). I could then use the second monitor when connected via the HDMI port.

Try switching to strictly using power saving mode and see if the dual monitor setup works though the system settings. If it does, then you know that you've got this quirky hardware issue. How the manufacturers solve this themselves in other OSes, and why is it not fixed in the Linux based drivers is beyond the scope of my knowledge.

But not all hope is lost. There maybe some other (better?) answers here [ Does Ubuntu 16.04 support hybrid graphics cards (bumblebee) ]. The current release (3.2.1) of bumblebee is rather old (2013) and I can't personally recommend installing it simply because I have not, but it seems that there are quite a few users that don't have issues with it. There is promise, however, of it's revival (checkout the develop branch on the Bumblebee Project githubs).

Whether you choose to take the bumblebee route or not, you should checkout the multi-monitor info on their wiki [ https://github.com/Bumblebee-Project/Bumblebee/wiki/Multi-monitor-setup ], it's quite informative.

I have this laptop and i use it with an external 4k screen connected via the thunderbolt(display port) socket. It works fine with both screens at 4k 60hz and i don't have an xorg.conf. I have noticed that the using HDMI port can slow things down a bit display port is much better.

I am using 16.04 which works great but i have not run 14.04 on this laptop but i used both the 15. releases stably as well.

I think the issue is that graphics driver is not the right one. I am using nvidia-364 from https://launchpad.net/~graphics-drivers/+archive/ubuntu/ppa and using the updates from there.

Incidentally this laptop and the owc thunderbolt dock work really well.