Classic puzzle, new approach or just a "two kinds of people" moment?

On the one hand, usual formulation is you only get one question, finding out which is the liar alone is easy but useless as then you're out of questions to actually get through the gate. On the other hand, unless you get the information about the behavior of the guards from a trusted source that isn't them you have no reason to believe them, and in fact they cannot relay the setup to you accurately without giving it away if you assume they're always like that as many do.

The full setup is that they can both tell you about the setup, but for the One Question, they follow the rules stipulated. Neither is actually bound to always lie and always tell the truth outside of the One Question.
- Well yeah you can make anything work when the rules change on a whim

Both guards told the truth in their first lines

Not if they were both lying. Both guards are liars.
- Which would mean guard 2 is telling the truth and not a liar
The way guard 2's first line is worded could be interpreted as him saying "the other (guard says) nothing but lies", meaning he's saying guard 1 is the liar which is a lie.

Sadistic DM Take - Both guards are liars and they came up with this rouse to fuck with people because they were bored.

Necro raises first guard, compels him to kill second guard.

Necro raises second guard, has them each explore the path they blocked to check for danger

Guard 2 says: "The other one nothing but lies", which is assumed to be true. From the guard that lies. Was that a lie?

I never liked this riddle. "Am I standing in front of you?" would tell you immediately
- To actually make it a puzzle, the trick is that you get only one question. Ever.
- The object isn't to identify the liar, it's to find out which path is safe. And you only get one question.
- Not much of a riddle anyway. You ask any guard what other guard would say and then negate that.
Well you should assume that both guards follow the rules all time. Meaning that the initial setup of the rules cannot be trusted, because who knows if the talking Guard is lieing or not. If one does tell nothing but the truth this must be the one explaining the rules.

However if I remember correctly it was setup that one will answer the truth while the other one will deceive me. Meaning he might tell the truth if it confused me.

So no question might work.
- Simple logic twisting really. Ask any guard what other guard would say, negate that answer and you got the right answer.

the guard: [sobbing] yes...

What if... they both only tell lies? No "one of us" about it, and it would totally mess with players' heads.

Ah, the Yu-Gi-Oh approach.

Barbarian street smarts ain't by the book (or all that smart), but damn are they effective

https://www.giantitp.com/comics/oots0327.html

Everytime I see the only truth and only lier situation, I gotta repost this clip:
https://youtu.be/i99jMtnE4vw?t=03m27s