Steps for creating a double exposure with the Bolex:
Check the footage counter before the first exposure, to see how far you will need to wind back the film. (You can also reset the frame counter to "0" in order to wind back to exactly same starting point in both exposures)
Film the first subject, compensating in your exposure by closing the lens an extra 2/3rd of a stop.
Check the footage counter at the end of the first exposure, to see how long to run the camera for the second exposure. (You may also determine this while filming by counting seconds on a watch or using the Bolex's internal audible counter.)
Close the variable shutter.
Release the spring by simultaneously turning the spring-disengage lever from "MOT" to "0" and the run switch to "M."
Insert the backwind crank into the 8 to 1 shaft and turn in the direction of the arrow (counter-clockwise), watching the footage counter to see when you've rewound to the beginning of the shot.
Before re-engaging the spring, first set the run switch to "STOP." (It is important that you do this before STEP 8, or else the camera will start running as soon as the spring is re-engaged.)
Re-engage the spring by turning the spring-disengage lever to "MOT."
Open the variable shutter.
Film second subject for the same amount of time as the first subject, once again compensating for the double exposure by closing the lens down an extra 2/3rd of a stop.
Don't forget to wind the spring before filming your second exposure!