Can two walk together except they be agreed? or, except they have agreed? The "two" are God's judgment and the prophet's word. These do, not coincide by mere chance, no more than two persons pursue in company the same end without previous agreement.
The prophet announces God's judgment because God has commissioned him; the prophet is of one mind with God, therefore the Lord is with him, and confirms his words. The application of the parables is seen in Amos 3:7, Amos 3:8.
The Septuagint, reading differently, has, "except they know one another."