>>415971836
The desktop window is already a mirror of the left eye of the main camera, so it's not really rendering twice. It's pushing a few more pixels to the screen by copying them from your main view, but a GTX 970 or above should be plenty to handle that.
VRChat currently uses 2 separate cameras to render, one for rendering your VR view and then another one to render it again but for JUST the UI. This is done so that the UI isn't affected by post-processing and can't be blocked by the world or an avatar.
Due to the nature of single-pass stereo, you don't exactly have 1 camera per eye. Once we switch to single pass stereo instanced rendering, we should see some performance improvements in terms of drawcalls.
Side note, if you open the handheld camera, that's +1 camera for the preview screen on the camera. Opening the stream camera is +2 cameras, one for the preview and one for the desktop stream.