Sounds interesting... but I haven't catch the idea, can you explain to me this optimization further, please? Thanks in advance.
The optimization comes from the old and simple concept... don't draw what you can't see. Now that gets a tad bit more complicated when 2 cameras are thrown in.
The other concept is that you cannot see a surface that is facing away from you.
Imagine for a moment that you were rendering a mirror (this is not a math correct method but...) you have a camera attached to the front of it and everything it sees, it draws onto the surface of the mirror. Now imagine that the mirror is turned away from you and you can no longer see the shiny surface, and therefore nothing that is being reflected by the mirror.
At that point, why on earth would you draw the scene from the mirrors presepctive (the reflection) if you cant see the reflection at all?
So, (and I'm not totally sure about the exact math functions for this, but I have a very dirty implementation of it) all you need to do is determine (per reflective object) what faces of the cube that the camera cannot see.
Now for a visual representation without resorting to MSpaint...
S--·--E <) = camera O = object
<) O do not draw east side
O (> do not draw west side
Unfortunatley, I can't give you any accurate math to use simply because mine is based off of 2D angles, IE only the side reflections can be occluded. It's not the optimal way to do it, but assuming you have a character which stays on the ground it usually works just as well.