Originally posted by vonBaloney -- so you just disengage and cock it without advancing, no rewinding needed.
Yes, but take up any slack first with the rewind knob and hold it in place to prevent any slight advance when re-cocking for the second shot.
It's helpful to plan on using any etched lines or split-image spots to assist in composing the two scenes.
If you take a whole roll of images rather than making an immediate overlay shot it helps to sketch and record each composition and note the exposure of each frame for future reference. Also carefully index the film when loading so it can be precisely re-positioned later.