That's an ancient (and very easy) riddle. The correct solution is to ask one bird what the other bird would say if asked which door leads to heaven.
Let's assume door 1 goes to hell, and door 2 goes to heaven. If you ask the truthful bird the question, he will say that the other bird would say door 1 (because the other bird is a liar, and would say door 1 goes to heaven). If the lying bird receives the question, he will say that the other bird would say door 1 (because he's lying about the other bird telling the truth, that door 2 goes to heaven).
Either way, you pick the other door.