A friend and I are planning a fun experiment. We'll each shoot a roll of film, rewind, swap, and then shoot over the other persons roll, creating two rolls of double exposures, and some hopefully interesting results. What I'm wondering is should we underexpose every shot by 1 f-stop so that they aren't all over exposed? Or is there some strange process I'm not aware of that'll work this out on it's own somehow? Thanks.