It appears that Unity has accidentally left out the “Stereo Non-Head Mounted” option in the Virtual Reality Supported settings in a Unity 2017.2 release. More info on how to work-around this here:

https://stackoverflow.com/questions/47286544/stereo-display-non-head-mounted-option-in-unity-2017-2-is-not-available

And it sounds like it will be re-added in a next release:

http://developer.zspace.com/answers/2167/unity-2017-2of3-virtual-reality-sdks

Let’s hope so!