Answer to paradox's question:

The question asked to both of them is, "what would the other person tell if I ask him the right way" (there is no other way to find out if they are lying or telling truth if you ask 'them' the right way)

The truthful guy will tell you that the other person would lie and show him the wrong way.
The liar will tell you that the other person would tell the truth and show him the same wrong way calling it 'right' the truthful guy showed you a moment ago.

That means they both showed you a wrong way, so take the other way.