Quite a classic one, I have to say.Just ask any one of them "Is the one telling lies with the box containing gold bar?"
If the answer is 'yes', go for the other box, otherwise, this is the box you want.
The key of this class of problem is to...
... make use of a paradox. You can derive answers to similar problems by constructing a paradox of appropriate reference.
Huasing Association 1999 - 2013