The method is already well known but I get some requests sometimes, so here´s
The current (first) picture has already been taken (fired) and we want to take a second exposure on the same frame.
1) Wind up the unused film tightly in the direction you normally would turn
the lever to rewing the film.
2) Hold the rewind lever you just wound up, to keep it from unwinding. Now
press the film release button.
3) Now wind up camera like you normally would to next frame while still holding
the rewind lever.
Upon winding up the camera, the rewind button pops out again, so you have to redo the procedure for every additional frame you want to take.