The one that always tells the truth cannot lie. The one that always lies cannot tell the truth. Once they are found out to be the truth teller and lie teller they will let the person inside and open the gate.
When asked questions both will answer in one way or another.
The one who tells the truth can only tell the truth and the one who lies can only tell lies.
They both anwser questions at the same time unless they have different answers.
They both look the same.