John Lightfoot on Matthew 18:19

[That if two of you shall agree upon earth, &c.] And these words do closely agree with those that went before: there the speech was concerning the apostles' determination in all things respecting men; here, concerning their grace and power of obtaining things from God.

I. [Two of you.] Hence Peter and John act jointly together among the Jews, Acts 2, 3, &c., and they act jointly among the Samaritans, Acts 8:14; and Paul and Barnabas among the Gentiles, Acts 13:2. This bond being broke by Barnabas, the Spirit is doubled as it were upon Paul.

II. [Agree together.] That is, to obtain something from God; which appears also from the following words, touching any thing that they shall ask: suppose, concerning conferring the Spirit by the imposition of hands, of doing this or that miracle, &c.