Haven't played with Unity's occlusion culling very much, but as far as I know it shouldn't be any different for a Rift-enabled Unity app. Sure, you've got two cameras, but each camera will render the scene in order (from lowest camera "Depth" to highest), and before each camera renders, it will go through the culling process (which may or may not involve occlusion areas you've set up) as it figures out what will be rendered for that camera.
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I am setting up occlusion areas at the moment. I am able to view them using Window>Occlusion > & checking Edit
I then tried adding a few simply objects to scene but they still appear even when outside the occlusion area.
I still have a lot to learn regarding setting up Occlusion culled areas. I was thinking of using LOD but this is more suited for swapping out objects with low Poly objects so occlusion culling should do the trick.