Since the focus of Starling is 2D, visibility is strictly a result of the order of objects inside the display list – so if a normal 2D "Sprite" object would occlude another 2D "Sprite" object, the same happens if you change them to "Sprite3D", regardless of the z position.
That's because depth tests would require the z-Buffer and bring a whole set of own problems (the z-Buffer doesn't know anything about translucent parts of a texture, for example).
So all you can do is handle visibility manually by sorting the objects differently in their parent. Or hide an object when it's viewed from the backside (that's what is done in the cube sample in the Starling demo).