On a Dell XPS 15, running Ubuntu 14.04, the HDMI and Displayport ports work fine, but when I plug in two screens to both the DP and HDMI, it doesn't work.
The screens window showed some error dialogs about bad configuration:
Could not switch the monitor configuration: could not set the configuration for CRTC 64 Failed to apply configuration: %s GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._gnome_2drr_2derror_2dquark.Code2: could not set the configuration for CRTC 64
When I force the second external screen "on" and click apply, the screens go all black except for a non-responsive cursor.
I tried with the Nvidia driver app in Nvidia (Performance mode), or Intel (Power Saving Mode). (after restarting). It still doesn't allow 2 external screens.
Update: it doesn't work under Windows with 3 displays either.