How Puzzling

“One of them always tells the truth, and one of them always lies.”
You’ve seen a puzzle like this before, but do you think it wise?
Oh, I can believe in the sort who lie so rarely that we can trust ‘em,
But liars prefer to include some truths to make it harder to bust ‘em.
And now that I think on it, how do we know the instructions are correct?
The instructor might be mistaken—or a liar, as I suspect.
And otherwise, it shouldn’t be hard to figure out who’s who.
Just ask for an answer that everyone knows, like the sum of two and two.
Then ask whoever has solved it right whatever you need to know.
These puzzles may be fun, but in real life, they’d have to go.