"The only time you get shadows that overlap with different 'darknesses' is when you have more than 1 light source."
Actually you can get overlapping shadows with a single light source if you're doing soft shadows.
Soft shadow effects also occur when you treat a light source as being more than a point source (E.g. a large glowing thing). Note that you get soft edged shadows in real life EVEN FROM A POINT SOURCE. Oh yeah, and objects tend to have bright spots in the middle of their shadows (the so-called "Poisson spot" predicted by a physicist named Poisson who wished to ridicule Fresnel's theory of optics and later verified by experiment), but they're usually not visible to the naked eye.
"BTW. in the real world out there you have Radiosity that could give you some kind of Shadow in the shadow nevertheless."
Radiosity essentially treats any lit surface as a light source so, yes, you get the issues inherent in having multiple light sources.
Actually you can get overlapping shadows with a single light source if you're doing soft shadows.
Soft shadow effects also occur when you treat a light source as being more than a point source (E.g. a large glowing thing). Note that you get soft edged shadows in real life EVEN FROM A POINT SOURCE. Oh yeah, and objects tend to have bright spots in the middle of their shadows (the so-called "Poisson spot" predicted by a physicist named Poisson who wished to ridicule Fresnel's theory of optics and later verified by experiment), but they're usually not visible to the naked eye.
"BTW. in the real world out there you have Radiosity that could give you some kind of Shadow in the shadow nevertheless."
Radiosity essentially treats any lit surface as a light source so, yes, you get the issues inherent in having multiple light sources.