This is actually my field of work. The composite method queermunist is referencing is the industry best practice for exterior hazard labeling. NFPA diamonds don't always or even often give first responders enough information to enter a building, so there's no utility to multiple diamonds. Responders really don't care how many chemicals are in a facility so much as what they are, and not many facilities actually using chemicals are set up in such a way that your example of encountering one chemical then another would work. They're just everywhere, even during normal operations due to distributed storage and distribution systems.

What these signs do is alert them to the degree of danger inside so they can make decisions, e.g., enter if just flammable, avoid water use, or (most common of all) to act as a reference to ask the building owner more questions before doing anything at all.