Look at it this way: a coin is a single thing. However, it's different (some would say opposite) on each side. So that would be one thing with two opposing features. Or similar. Like a character that adores kittens but is a violent fighter.
The saying usually refers to a good and a bad thing having a similar cause because they're related, in a way that makes them impossible to separate... which means you'll have to accept that there's side effects to the good thing... and often also in a sense that what is bad or good is subjective. And in the general sense, it can refer to two things that appear to be very different but really are the same if you dig deeper.
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