If you press the film release button like you are going to rewind exposed film you may be able to activate the film advance lever to cock the camera so you can take another picture, and the film should stay on the first exposure allowing you to re - expose it. I would underexpose each image 1 F stop because.you are exposing the same film twice. Double exposures are always an iffy proposition and anything can happen. After the second exposure cocking the film advance should allow the film to advance once again. You might test this on the final pictures on a roll in case the camera won't start advancing the film after you try it. This way you won't blow a whole roll worth of pictures testing it.