Overlapping shadows

You'd prefer it expressed mathematically?
Of course you would!
Consider a plane illuminated by two light sources - A and B.
Consider also a function, FA, which has a value for each point on the plane. If a point has a clear line of sight to light source A, then the value of FA at that point is 0, indicating that it has no shadow. Whereas, if the point has no clear line of sight to light source A, then the value of FA at that point is 1, indicating a shadow.
Function B is similarly defined with relation to light source B.
Then the function which describes whether each point is in an area of complete shadow (i.e. is shaded with respect to light sources A and B) is simply FA x FB.

Or, to put it another way, I can see a little, dancing man.