combined cameras

Is it possible to combine more camera views in the same scene7same time - for example overlaying a perspective view with an orthographic?

If you want overlay your perspective camera's view with your orthographic camera's view, there is a simple way to do that. Just set your orthographic camera's depth value higher than that of your perspective camera's. If you want to learn more about Unity cameras in general, I recommend efge's link to the Unity manual.

Take a look at the "Normalized Viewport Rectangle" section of the Reference Manual for the camera object. Orthographic/Perspective can then be toggled for each individual camera.