Since, it's just me and you in this: You HAVE to get rid of the first sentence. Until you do that, we'll keep going round and round.
You ONLY know:
a. They're twins
b. One lies and one tells the truth.
c. Each twin knows which door they're guarding.
THAT'S. IT.
Anything else, and one is trying to make their argument based on something that's NOT in the original scenario.
That's all I'm saying. 
I see what you're saying. Try it this way...
Let's say that we have
Twin A and
Twin B. We'll assume we don't know who's the liar or what door they are guarding.
Here's the coversation.
Doubles: "Say,
Twin A, if I go ask your boy over there which one of these doors leads to heaven, which one will he say is the Heaven door?"
Twin A: "He'll tell you that my door is the Heaven door.
1. If he's the lying twin, he's lying about what
Twin B would say because
Twin B would tell the truth and say his own door was the Heaven door.
2. If he's the truthful twin, he's truthfully stating that
Twin B will lie by saying that his own door is the Heaven door.
Therefore, which ever door is stated, go to the opposite door.