I doubt that it is a simple on/off switch as such a fitment would normally be built into the lamp holder or be part of the fixed installation.
One possibility is that the lamp contains two filaments of very different outputs. One dim for economical night lighting in say a sick room, or in a toilet, the other being much brighter for general use.
Another possibility is that the lamp contains two similar filaments, with the intention that when one filament burns out that the second filament may be used. The idea being that a twin filament bulb gave twice the life for less than twice the cost.
Never popular because they did cost almost twice as much as two standard bulbs, and the glass bulb was blackened to a greater degree and output on the second filament thereby reduced.
