What goes on the NFPA 704 for multiple chemicals?
According to Section 4.2.3.3 of NFPA 704:  


  Where more than one chemical is present in a building or specific
  area, professional judgment shall be exercised to indicate ratings
  using the following methods:  
  
  (1) Composite Method. Where many chemicals are present, a single sign
  shall summarize the maximum ratings contributed by the material(s) in
  each category and the special hazard category for the building and/or
  the area.  
  
  (2) Individual Method.  Where only a few chemicals are present or
  where only a few chemicals are of concern to emergency responders
  (taking into account factors including physical form, hazard rating,
  and quantity), individual signs shall be displayed. The chemical name
  shall be displayed below each sign.  
  
  (3) Composite–Individual Combined Method. A single sign shall be used
  to summarize the ratings via the Composite Method for buildings or
  other areas containing numerous chemicals. Signs based on the
  Individual Method shall be used for rooms or smaller areas within the
  building containing small numbers of chemicals.