How to render in a camera just the visible polys in another one camera

Hi,

First than all thanks for your time, now here we go. Well imagine a scene with two cameras, the first one a camera who emulate the vision of the player (like a FPS perspective,) and a second camera (Isometric perspective), ok now my question: how i could mark the visible polygons(not the Objects, just the visible polys) in the first camera to then, render them in the second (Isometric/Main Camera)

I don't want to render in the second one what the first one sees, like RenderTexture, my game is an Isometric view game, but i don't want for example that the player can sees behind him, just what is visible for his "eyes"(first cam/fps cam)