You have to know what the other constituent metal was, otherwise all you can say is one is more dense than the other.

If, for example, one component was tin, and for all intents and purposes the only other component was lead, then you could figure out the percentage of each in the alloy pretty easily. Because we know the density of tin, and we know the density of lead.

It’s what is called a weighted average.

But it gets considerably harder, if there are multiple alloying components.