The amount of overdraw doesn't change with the number of cameras. Its the number of times you draw on a particular pixel, regardless of how you do it.
Well, in your case it isn't that bad really - you're not gonna get an insane amount of overdraw, unless everything is huge and everything is drawn on top of each other covering large amounts of screen. Small amounts of overdraw is fine, its when you're overdrawing the full screen many times is when things go horribly wrong.
For the clouds, mountains, etc, use the dicing feature to save on fillrate, or maybe use the custom mesh stuff to tightly draw a silhouette around the visible parts of the shape.