I would ask any one of the two guard this question:
"If I ask the OTHER guard, which door leads to Heaven, What would be HIS answer?"
Whatever the answer the guard (you asked) gives, take the other door to reach Heaven.
Logic is:
1)Assume G1 guard is truthful and is guarding Heaven and G2 is liar guarding Hell.
If you ask G1 what G2 will say if asked the door to heaven, G1 will truthfully tell you that G2 will lie and tell that the door G2 is guarding leads to heaven. Take the opposite door that is the one G1 is guarding to heaven.
2)Assume G1 (truthful) is guarding Hell and G2(liar) is guarding heaven. If you ask G1 now, G1 will tell that G2 will lie and tell that G1 is guarding the door to heaven. Take the opposite door the one guarded by G2 to reach heaven.