This is the level consisting of planes as map entities. Both views, camera and camera 2 start at the origin (0,0,0) looking in the same direction.

If only the main camera is turned on, everything is ok. The plane entities are visible and within the clipping range.
By pressing 'T' the second camera becomes visible. As you can see the second camera has a different clipping range but suddenly the plane entities are gone on the main camera. Using different clipping values on camera2 changes the clipping range of the default camera.
If you want you can download the
project exe to check the problem.