I have a MacBook Pro (15-inch, Mid 2010) which under OS X reports a model
identifier of "MacBookPro6,2".  This laptop has two GPUs.  The integrated Intel
GPU and a discreet Nvidia GPU.  Under OS X there is some magic that I don't
understand to allow the laptop display to switch between these GPUs.

Under Linux these GPUs show up separately to X, each reporting a connected
display.  Only one is actually controlling the display of course.  With a clean
default install of openSUSE Leap 15.0 the default behaviour was for the desktop
to be extended across the these two apparent connected displays, which resulted
in me only being able to see half of the desktop.

I had to open the "Displays - System Settings Module" and disable one of the
displays in order to see the full desktop.  (Interestingly this settings panel
doesn't allow mirroring the displays.)

When I use Ubuntu on this same machine it too reports there being two GPUs with
one monitor each, but at least the desktop environment defaults to mirroring
across all monitors rather than extending; meaning that out-of-the-box usage is
just fine.