The reason for listing them separately is because each individual chemical has its own ratings. You can’t simply take the highest of each and combine them into a single sign. For instance, in this case one chemical isn’t flammable but is explosive when heated. The other chemical is flammable but not explosive. So if you see a chemical on fire, you know it’s the second chemical and isn’t explosive. But if you see something that isn’t burning in a room full of fire, you know it’s a potential powder keg waiting to explode.

Okay, so the two signs on the building have a weird combination.

The sign on the left indicates something that isn’t flammable, but reacts with water. The sign on the right indicates something that is flammable, but there’s no risk of reacting to water. If the building caught fire then a first responder on the scene has to read both signs at the same time. They can’t spray the building with water because the non-flammable substance would react with the water.

So why aren’t the signs combined? They have to be treated the same anyway.