Unless the sirens can produce different sounds that are easily distinguishable, then you can't have one alarm with two meanings when the meanings are absolutely not the same. If you hear the siren, how do you know if you need to pack your things to get out of the way of a fire or to seek higher ground from a tsunami where both of them might not be visible to you at the time of the siren?

Unless the response to the siren is the exact same thing, you can't use the siren for more than one thing.