Hilchos Rechilus, chapter 6, section 5 6

The ban on accepting rechilus applies even if he heard it from a person he trusts as much as two witnesses — and even if there is no way at all to judge favorably.

All this is when there is no benefit for the future. But if there is a benefit — so he knows how to protect himself in the future — he may accept it and believe it. He may not tell it to others, unless telling them is also for a benefit.

All this is when the person told him what he saw himself. But if he said he heard it from others, there is no permission to believe him. He is like any other person who told him a story.

And this whole permission is only for a person he knows well and always believes as much as two witnesses. But if he does not believe him about other things, and only finds it convenient to believe him about rechilus, he is surely breaking the ban on accepting lashon hara and rechilus.