Albert Barnes on Matthew 18:19

Again I say unto you, That if two of you, etc. This is connected with the previous verses. The connexion is this: The obstinate man is to be excluded from the church, Mt 18:17. The care of the church--the power of admitting or excluding members--of organizing and establishing it--is committed to you, the apostles, Mt 18:18. Yet there is not need of the whole to give validity to the transaction. When two of you agree, or have the same mind, feelings, and opinion, about the arrangement of affairs in the church, or about things desired for its welfare, and shall ask of God, it shall be done for them. See Acts 1:14-26, 15:1-29. The promise here has respect to the apostles in organizing the church. It cannot, with any propriety, be applied to the ordinary prayers of believers. Other promises are made to them, and it is true that the prayer of faith will be answered; but that is not the truth taught here.

(q) "it shall be done" Mk 11:24, Jn 16:24, 1Jn 5:14