Display all scene cameras in another camera?

Hi all,

my game view is currently 'made up' of multiple cameras that render the terrain, the buildings and the scenery necessarily separately.

So in the game view you see the scene as it 'should' be, but in reality all cameras see only 'a bit' of it.

What I need is a camera or any other mean to 'capture' or 'display' the game view in a single camera, so that I can apply an effect to it.

All this from the fact that if I want to apply a scene-wide post rendering effect, I'd need it on all cameras, which is a burden and certainly more resource intensive than applying to a single camera.

Ideally, what should happen is that if I turn off this 'uber-camera', the game view goes 'blank', while to achieve that now I should turn off all off the individual cameras one by one.

EDIT: problem is, the 'render bits' and their relative cameras are displaced all over the scene, so it's impossible for a single camera to 'see' the 'composed' game scene as it should be.