It's been a long day, so this may not come out right. BUT

The person you ask is A telling the truth B lying

The other person guards C the door to hell D to heaven

And you can pick either person to ask, this gives 4 possible combinations.

What you do know, by asking what the 'other one' would say you are getting a two layered answer BUT Exactly one layer is a lie. And You are thinking about going into Door A, bt asking about door B

Option A, Door A, To heaven, honest answer. Door B is to hell, but is guarded by a liar, so first layer of answer is Yes -this is the door to heaven. The person you are speaking to will not alter the reply - so will say - Yes You are at the door to heaven & heard yes - go in.

Option B Door A, To heaven, speaking to the liar. Door B - Hell - honest twin. First layer of answer - No-this isn't the door to heaven. But the liar at door A lies & says Yes. You are at the door to heaven, you've heard yes - go in.

Option C Door A- Hell, with the liar. Door B, Heaven & honest twin who would say Yes, but the liar you actually spoke to says No. You're at the door to heel, heard NO, don't go in.

Option D Door A -Hell with the honest twin, Door B - Heaven with the liar - who would say No- you're not at the gate to heaven. The honest twin you are speaking to reports it accurately as NO, so don't go in.

Like JohninKansas said

Ask - would the person at the OTHER door tell me if I asked them if that was the door to heaven. If they said YES - go in If the said NO - don't If you only ask about your own door, you can't know if it's true or a lie. But by putting the answer through BOTH twins you know you are getting exactly one lie, invert that you get the truth, guaranteed.

Hope that makes some sense !

What's next Mrzzy ???

Hilary