Dual layer DVD question
The lenses used by those DVD players have an extraordinarily shallow "depth of focus". They can easily focus on the inner layer and the outer layer will be WAY out of focus.  It is easier to do this with shorter wavelengths of light (toward blue and ultra-violet).  This is one of the reasons why the HD version of DVD uses "Blue-Ray" or shorter-wavelength light.  

Commercial versions of dual-layer discs are moulded as two separate layers with different data patterns, and then laminated together. Field-recordable discs are written by focusing the lens on either the inner or outer layer.