Multiple exposures with an M?
Aside from the Leica M5, which is the only M to have built-in support for multiple exposures, for all other Ms — forget it. Although you may think you can press down the rewind lever to disengage the film-wind, it doesn't work. What will happen is that when you pull on the film advance lever, the rewind lever will pop back.
What newbies then try is to physically force down the rewind lever to prevent it from popping back during advance. Uh-huh… Hear that tearing, grinding sound inside your camera? That's the sound of the wind-on mechanism getting trashed and gear wheels being stripped or snapped right off.
So… do not attempt to shoot multiple exposures with an M camera by holding down the rewind lever. It doesn't work. Forget it.
The only safe and non-destructive way to do multiple exposures is to use a flash with the shutter set on "B". Since the shutter never closes, you never wind the film and thus can make as many exposures as you like.