a-gre' (sumphoneo, "to be of the same mind," "to come to a mutual understanding"): This is the sense of the word in Mt 20:2; Joh 9:22, and other passages. In Mr 14:56 the word is isos and has the thought not only that their words did not agree, but also that the testimony was not in agreement with or equal to what the law required in such a case. The thought of being equal occurs also in 1Jo 5:8.
⇒See the definition of agree in the KJV Dictionary
The figurative use of the word in Mt 18:19 makes it of special interest. The word there is sumphoneo, from which comes our word symphony, meaning a harmonious blending. This agreement therefore is complete. Three persons are introduced: two human beings and the Father. They are in perfect agreement on the subject or purpose under consideration. It is therefore an inward unity produced by the Holy Spirit leading the two into such an agreement with the Father. There will follow then, as a matter of course, what is promised in Mt 18:19-20. In Ac 5:9 it sets forth the justice of Peter in dealing in the same manner in both cases. Ananias and Sapphira were in perfect agreement and equally guilty (Lu 5:36; Ac 15:15).
Jacob W. Kapp