Using both PFD_DRAW_TO_BITMAP & PFD_DRAW_TO_WINDOW

I know there are PixelFormatDescriptors that have both PFD_DRAW_TO_BITMAP and PFD_DRAW_TO_WINDOW flags set. However I cannot find a way to switch between the screen and the bitmap. Is there a way to dynamically switch where openGl is drawing to or is this not now the 2 flags work when they are both set.