Hilchos Rechilus, chapter 9, section 5

Now we will explain another big rule in these matters. It will also explain the fifth condition above.

Know that the whole permission to tell, which we wrote above, is only if the person he is telling has not yet closed the deal with the one he is telling about. But if they already closed the deal, it depends.

If he knows that because of his words the listener will not harm the one he told about — he will only know how to protect himself from harm — and all the conditions are met, then it is allowed, and right, to tell him.

But he may know the nature of the listener: if he tells him, he will surely accept it as the full truth, judge the case by himself, and act on it — cancel the partnership or do other harm because of his words. Then it is forbidden, even if the listener will not do more than a beis din (Jewish court) would have ruled if two witnesses had testified to his words.

The reason: by his words he causes the person more than a beis din would have ruled against him. A beis din would not have ruled against him based on his words, because he is only one witness.

Based on this, it seems that if there are two people telling, and they saw the thing themselves, they may tell — if their only goal is to prevent harm, if it will not cause more harm than a beis din would have caused, and if all the other details are also met.