Two questions and no trickery:

Ask the 1st guard "Does your door lead to certain death?"
Ask the 2nd guard "Does the 1st guard's door lead to certain death?"

Take these actions depending on their answers:

Yes and yes, they are both telling the truth, use a different door
Yes and no, one of them is telling the truth, use a different door
No and yes, one of them is telling the truth, use a different door
No and no, both are telling the truth, use that door

Two questions and no trickery:

Ask the 1st guard "Does your door lead to certain death?"
Ask the 2nd guard "Does the 1st guard's door lead to certain death?"

Take these actions depending on their answers:

1. Yes and yes, they are both telling the truth, use a different door.
2. Yes and no, one of them is telling the truth, use a different door.
3. No and yes, one of them is telling the truth, use a different door.
4. No and no, both are telling the truth, use that door.