Kent is right on track. The "light-out" indicator measures resistance in each circuit. So if one bulb is burned out and the other is not, the indicator illuminates. If both are burned out, the resistance is the same so the indicator light does not illuminate.

Concerning your practice of using the fog lights to provide more light, I don't think you are achieving any substantial benefit under normal conditions. The fog lights throw their light low, wide, and not very far. I don't think they add very much light down the road in clear conditions. However, I would like to hear from others on this.

regards,
Mark