I worked for pre-merger NW in the summer of 2008, and we had to go through a standard training program before we could work; something all employees did annually (even if the position didn't come close to needing that information, like mine). This exact question came up, of what to do if a passenger with dog allergies was on the same flight as a blind pax with a guide dog. The correct multiple choice answer was that the pax with allergies would have to be put on a different flight, as an allergy-free flight couldn't be guaranteed or something like that (not to mention ADA rules that I'm sure would come into play)