Want Main Camera to show what fxcamera is rendering

I have two camera in my project. the main camera which renders the whole scene aside a layer which is a Fx layer and then an animated camera that slowly renders a cutted sphere that is a bit flat( the idea is to slowly render it from above looking like the material on it is slowly expanding into the whole scene). this last one renders only the fxLayer that i mentioned. The fx camera is a child to the main camera. what i want is for the main camera to visualize what the fx camera is rendering. i want to learn this technique because i saw many a vfx artist use this trick with interesting result. the image you see is a test