Then, rip both of them in half and knock down the safe door so that everyone after you immediately knows the safe route

The Barbarian got what they wanted, which is to have an excuse to rip another head off.

If you rip them both in half, then two of your party are cursed to be the next two truth/lie guards. Roll for unintended consequences.

Time to rip the table, the DM, and everyone's minifigs in half. It's rippening time.

Opening the certain death door reveals a guy in a dark robe with a scythe: "Hey, what's up?"

*[sings]:* I'd like to rip the world in half / for perfect disharmonyyyy!

For years, I had my own headcanon for the Labyrinth movie. In the scene, the young Sarah correctly solves the riddle, passes through the correct door, says "This is a piece of cake!" and then she immediately falls down a pit of doom. This confused me, because she got the answer right, so I reasoned that the guards were both liars, and because they both participated in explaining the rules, they were lying about the rules. It was only a few years ago that I read in an interview that the Labyrinth dropped her down the hole because she said it was a piece of cake. It was her arrogance that set her back, not that she got the riddle wrong. But now it still bothers me that the liar, whichever one he is, helps explain the rules of the scenario. If he *always* lies, then she can't trust that either of them ever tells the truth. The rules have to be described separately, like on a sign or by a disinterested third party. Or you could phrase it differently, like "One of us will answer your question truthfully, and one of us will answer your question dishonestly." That way you avoid saying that they *always* lie, and specify that the lie will only be in response to the one question. Fuck, I've had too much coffee. How the fuck did I get up on this soapbox? Why are you still reading? Go do something productive.

The guard replies "I don't know for sure".

Alternate solution: 

> Go do something productive. No.

“So, you’re telling me I could have just greater restoration’d the guards rather than killing them? My god isn’t going to like this.”

Ah. Normally I see this with no limit on questions. You're right. It'd only work with at least two questions.

I'm a fan of the revised Little Mermaid song: https://youtube.com/watch?v=fcbazH6aE2g

I've only heard it with one question, that's the whole point. Otherwise you just ask a guard some trivial question (e.g. What color is the sky?) to determine which is the liar, then just ask which is the safe door. The whole point is to get the information you need from a single question.

Maybe I've only seen a fucked up version.

*PLEASE BE QUICK ABOUT IT, I HAVE A LOT OF WORK TO GET TO.* "Uhh.... Wrong door, sorry."

This puzzle is always presented as difficult, but why not just ask a known? If your eyes are brown just ask “Are my eyes brown?” You’d immediately know which one lies or tells the truth.

The difficulty comes from only being able to ask one question. It's very easy to figure out the liar, but it's much more difficult to figure out the liar and the correct door in the same question

That assumes the other guy holds to his principles in the face of death. If I were the dm, the act of tearing the other guy's head off and then threatening to do the same to the other one unless granted another question would at least grant advantage on an intimidation check

Because there are two doors and only one question. If you ask a known question unrelated to the door you find out who the liar is but lose your opportunity to ask them which is the correct door.