Multiple cameras will each render what they see. However, unless you limit them to portions of the screen (called viewports) one will always render on top of the other. Which renders first and which last is determined by the depth, not by which one is tagged as the main camera.
To switch from one camera to the other, try setting enabled=false on one of them.
A simple example of rendering with two cameras to different parts of the screen can be found here.
To overlay two camera views (for example if one is generating the background) see the documentation on Camera.clearFlags.