I agree. Owslachiefs answer is also "correct" in that it will give you the information you seek in this particular problem. The problem statement gives you 3 key pieces of information:
1) there is a truth teller
2) there is a liar
3) the truth teller is in front of the heaven door (so the liar is in front of the other).
Given those 3 pieces of info, owslachef's answer works perfectly. I've seen a similar problem statement that omitted item #3 above: the guardians are just standing together, off to the side, and not associated with either door. In that case, owslachef's answer doesn't work - his question will tell you which is the liar and which is the truth teller, but then you would need to ask a second question to find out which door to choose, as that information is not "linked" to which guardian is which. The answer I gave works whether the guardians are associated with the doors or not.
Interestingly, if the guardians are not assigned to particular doors and you ask the question I gave, you would be able to choose the correct door, but you would still *not* be able to identify which guardian was the liar.