Background/overlay scenes (not layers) : Possible in Unity?

I currently use layers to separate objects, display several backgrounds (parallax scrolling) and overlays.

However, separate scenes would give me a better control on Render settings per scene : Ambient light, fog, etc... And surely more than I'm aware of.

  1. Are background/overlay scenes (not layers) possible in Unity?

  2. Apart from Render settings, what would be scenes advantages over layers?

Thank you.

You cannot have multiple scenes overlapping like you suggest. So, no they are not possible, and no there are no advantages. You have to use layers, you could loadleveladditive, but that would load one level behind the next, not create a background effect.