Composition : See scene 1 through a lens, and scene 2 outside

Question

I need to display a monochromatic and blurred world. Then a pair of glasses pops up and the world appears colored through them. It finally covers everything. All materials are controlled by a set of World custom properties. This variables range from 0 (monochromatic) to 1 (colored). I guess I will use the compositor, but I am not sure how to configure it : I will have to copy the main colored scene to get a monochromatic one. It means I must be able to change my custom properties to zero for this new scene. But if i edit the camera animation in scene 1, I would like it to be synchronized in scene 2. And in the composition process, how can I display the colored scene through the lenses and monochromatic scene elsewhere ? T H A N K S !