Even Peter had come to realize this through the events involving Cornelius and his house, and had to be rebuked for behaving in a way that was inconsistent with what he’d learned through these events (and which was inconsistent with the truth of the evangel Paul had been heralding among the nations). And if this was something recognized by Peter (as well as James and John), then what the Galatians were being pressured by certain Judaizers to do (i.e., proselytize to Judaism) was inconsistent with not only Paul’s administration and ministry, but with Peter’s as well. However, it should be emphasized that Peter’s realization that those among the nations didn’t have to proselytize to Judaism in order to be saved is a far cry from saying that, like Paul and Barnabas, he had come to perceive the much greater (and more recently revealed) truth that Paul went on to defend in this letter, concerning the righteousness that is based on Christ’s faith alone rather than on anything that we do or don’t do (including any and all “works of righteousness”; see Titus 3:4-7).
Excluding Peter from Paul’s “we” in Gal. 2:15 is, therefore, not some “ad hoc” move on the part of those who affirm the “two evangels” doctrinal position. Rather, it’s an interpretation of what Paul wrote that is informed by the broader context of Galatians, and is in keeping with what we know to be true about the two distinct apostolic ministries and administrations that belonged to Paul and Peter, respectively. Paul (and his apostolic companion at the time, Barnabas) had been severed to God from the rest of the apostles for a distinct ministry that was in accord with a new dispensation and administration (Acts 13:1-3; 20:24; Eph. 3:1, 9). And the truth that they dispensed among the nations (and those Israelites who came to faith in Christ through their ministry rather than before the start of that new administration) was truth that I see no reason to believe or assume that Peter, James and John had come to embrace, or had begun teaching the Jews, proselytes and God-fearers within the sphere of their “for the Circumcision” ministry.