Be careful when using the words two gospels. As I indicated in my previous post words mean something. None of us here would argue that the rules changed if you will from the Old Testament to the New Testament. Is that to be considered two Gospels? Of course not. Because of Adams sin God had to buy back or redeem the world if you will. God changed the way He dealt with not only Israel but the Gentiles as well.
In a similar way there is a difference after Christ ascended and the Holy Spirit was sent to indwell all believers. The nation of Israel rejected the Messiah which caused God to set them aside and through grace give the Gentiles a joint heir status to them. The rules changed once more. I believe that no one can argue that Paul was given a special role to play in dealing with the Gentiles. In that change faith without the works of the law became the rule. Add anything to that faith and you risk adding works which Paul said would be making grace no longer grace.
Rom. 11:6, "But if it is by grace, it is no longer on the basis of works, otherwise grace is no longer grace."
He also said that if we add anything to saving faith through grace we nullify the grace of God.
Gal. 2:21, I do not nullify the grace of God; for if righteousness comes through the Law, then Christ died needlessly.
There was definitely a change with the teachings of Paul
Peter, the 11 and Paul all preach the same gospel as clearly outlined in I Corinthians 15. Jesus Christ crucified, Blood shed, died, buried and Resurrected on the third day and now at the Right Hand of the Father. Where Paul is the master theologian and communicator via the 'pen', what we have mostly in Acts for Peter are sermons calling sinners to repentance and receiving Christ Jesus. Peter's epistles match Paul's although styles, understandably, are different. The message is the same. We even see Philip use Isaiah 53 to witness to the Ethiopian Eunuch.
We all quote Paul a lot more than the others because he obviously wrote more and mostly to predominately Gentile churches.