ANSWER: "If I asked the other person, 'Does the left door lead to the riches?' what would the response be?"

EXPLANATION:
To guarantee a lying response, bring both people into the question's response. This ensures that with one person always telling the truth and one always lying, the answer has to be a lie. Ask either person "If I asked the other person, 'Does the left door lead to the riches?', what would the response be?"

To guarantee a truthful response, ask "If I asked the other person, 'Does the left door lead to the riches?' would they say 'no?'"