The Chofetz Chaim writes that if one hears lashon hara and the information may be of some purpose to the listener – the listener can only suspect it, and not entirely believe the information, until he has clarified it as being true.
How would one clarify such information without accepting loshon hora. At any step along the way of clarifying – the 2nd or 3rd person and so on may also just be speaking loshon hora.
As soon as a person hears the information from two witnesses, he can certainly believe it as being true. This would presumably be the intention of the Chofetz Chaim.