[HTC Vive] Mix Reality using Steam VR without the third controller

I read this forum in the steam : Mix Reality Videos

It said that :

"The new Unity plugin (v1.0.8) will automatically create this for you given two conditions:

1) Add a file called externalcamera.cfg in the root of your project.

2) Attach a third controller to your system."

So basically, without the third controller it is impossible to develop a VR game that supports Mix Reality ? Sorry if this question looks like stupid question but I am a beginner in VR and still learning to develop one for HTC Vive.

Thank you.