Why do we use the plural "heads" and "tails" when describing sides of a coin?

The Oxford English Dictionary has one citation from 1801 which puts it in the singular, but the earliest citation, from 1684, has ‘heads or tails’. I think we must regard heads and tails,when found in this context, as examples of ‘pluralia tantum’, the term used to describe nouns that end in -s, but whose meaning is ‘collective or composite’. Other examples are dregs, thanks and remains.

Surprisingly, no one has mentioned about the metaphor here. (cf.Wikipedia, I suspect that it is properly called a metonymy.)

In expressions like heads and tails, we really are not referring to the literal head or tail on a coin. The symbolism is merely a convenient way of referring to the obverse and reverse sides of the coin.

We generally use the plural when a metaphor or a symbolism is used this way to refer to a certain class of associated things . So, this is not unique to heads and tails alone.