problem with fog lamps

Marcel!

My bus has front and rear fog lamps. Now both of them are working almost* correctly. ( * the rear lamp can be switch on when the front is not lightning)
I made that the fogs lamps are can be turn on when the dimmer is on. The problem is when the fog lamps are on,and i push 2 times the button Shift-L every lights turn off. But then i cannot switch back with the button L, just when i switch off the fog lamps. I watched the original SD202 and the situation is the same. I cannot switch on the light as long as the rear fog light is on, BUT with the mouse i can rotate the key into the second position and every lights turn on.

This is a bug or a feature?