Adam Clarke on Numbers 35:30

But one witness shall not testify against any - This was a just and necessary provision. One may be mistaken, or so violently prejudiced as to impose even on his own judgment, or so wicked as to endeavor through malice to compass the life of his neighbor: but it is not likely that two or more should be of this kind; and even were they, their separate examination would lead to a discovery of the truth, and to their conviction.