That's funny! but if you want to know how to solve this problem every time, even when asking one single question, just ask this question:

"If I ask the other guy which is the correct path, which path will he tell me?"

No matter who you ask, both of them will point to the WRONG path, meaning the correct one is the one they DIDN'T point to. Here is the logic.

For the sake of argument, let's assume the correct path is the right path. When you ask that question, if the person is the truthful one, he will be honest and say the left path. Because if you ask the liar what the correct path is, he will say it is the left path (which is false). Now if you ask the liar what the other guy will say the correct path is, he will lie to you and say it is the left path (which is also false, the truthful one will tell you it is the right path and not the left).