I'm going by memory of a long-ago article in 'Modern Photography,' I think, so NO GUARANTEES... but I believe it said that you can make a double exposure by taking up the film tension with the rewind knob, holding it in place, then pressing down the rewind lever and holding it as you stroke the film-advance lever.
However, I do vividly recall that this tip ended with, "There will be a horrible gnashing of gears, but Leitz says no damage will occur." Well, maybe that's what they said then... now, who knows?