Randall Ellis , May 05, 2008; 12:05 p.m.
You simply expose two negatives onto the same piece of paper one after the other. In many cases part of each exposure will be masked off (beware of hard edges though) to create a montage of images. Imogen Cunningham did this quite a lot for a time if you would like to see examples. This was also regularly done before panchromatic emulsions were developed (ouch!) - due to the color of the sky and the color sensitivity range the sky is devoid of detail with orthochromatic emulsions, so during printing a second negative exposure of just the sky would be printed down onto the paper after the first negative was printed so that the resulting print would have clouds or a moon for example in the sky.