Two lights uses light from two separate sources for more even lighting effects
The lighting model has two different modes. With Depth Cue turned off, the lighting model uses light vectors (or directions). In this mode, the shading is not affected by the position of the light, but rather only the direction of the light. In this case you can specify the two light vectors as Light Vector 1 and Light Vector 2.
With Depth Cue turned on, the lighting model uses lighting positions. Both the lighting direction and distance from the surface to the light affect the shading of a surface. In this case you can specify the two light vectors as Light Position 1 and Light Position 2. The amount that the light is diminished is determined by the following formula:
1/(K + Linear * Distance of surface to light)
Both K and Linear variables are available for you to alter. Setting K or Linear to larger values this will darken the picture.