The question is; What question do you ask what girl?

since you can ask them both a question .. ask one did my girlfriend walk by, if the answer is yes that is the one telling the truth if not its the liar, and the second one ask which way did she go the answer to this one depends on the last. if the last answer was yes you go down the path that this one doesnt point to, if the last answer was no then you go down the path that this twin points to