This happens because the absence of lights correspond in fact to the existence of a light with Ambient component set to (1.0, 1.0, 1.0), and Diffuse, Specular components set to (0.0, 0.0, 0.0), the maximum luminosity that can be achieved. As the light components of the two lights are added, the total ambient light is now (1.0, 1.0, 1.0), so the maximum luminosity has been already reached and additional diffuse light cannot increase further the luminosity.