Between ISO 400 and 640, there is not even one stop difference. (0.8) Unless we are talking about extreme low light conditions (which one stop difference isn't likely to solve), I wonder why there is a need to switch between the two.
How about simply loading ISO 400 film in one camera and higher ISO one in your second/backup body? Fuji has an ISO1600 and Kodak has an ISO800 listed as current offerings.
I am finding, NOT using flash will make it very difficult to get the right catch light in subject's eyes and eye socket part of the face tends to have deep shadows making the result unattractive. I am not sure if "getting this right" is possible in fast pace shooting condition like weddings where things need to proceed quickly and there isn't often much of a choice to play with subject placements.