Ask "What would the other person say if I asked him which way to go?"

Correct! or close enough...

The full explanation:

Which cave will the other person tell me to take? The person telling the truth will tell you the wrong tunnel, because that's the one the liar will tell you. The liar will tell you the wrong tunnel because that's not the one the person telling the truth would tell you. So, no matter which one answers, they will tell you the wrong tunnel. You just take the other one.