The contrast throughout Galatians is between those Gentiles who had become Christians, and those Jews who taught that they (the Gentile Christians) must be circumcised and "keep the law" to be saved. There is evidence here and in Colossians of syncretistic gnostic Jews attempting to subvert apostolic Christianity, but the primary dilemma or choice if you will facing these Galatian believers is some form of Judaism vs Christian, apostolic faith.
Therefore it seems more likely that the "two covenants and the two "Jerusalems" and the two "children of Abraham" and the two "mothers" represent that same dichotomy. Both women and both children had a connection to Abraham, but one was temporary, based on bondage, and according to mere flesh. The other was age-lasting, based in freedom, and according to the Divine Word of Promise effected via the Spirit.
The two Jerusalem's are likewise both cities claiming covenanted status with God as the place where His Name is located, but one was in bondage and a persecutor of the other who was free and being persecuted.
So it seems that the two covenants most likely are the old, and the new. The old had been broken by Israel (hence God ordained a new one) but the remnant of Judah and her companions were clinging to the old marriage, and demanding the nations join it as well. But that would be like living after a divorce as if there had been no divorce. Lawfully, the marriage was severed and ended, so the woman could be married to another (the resurrected Christ) - see Paul's argument throughout
Romans 6-7.
So, again, the issue seems to be "what is the basis of your covenanted status with God? Is it fleshly circumcision and adherance to the Sinaitic Covenant? Or is it spiritual circumcision of the heart in Christ?"
Note, there is nothing in Paul's argument(s) to suggest obedience to the commands of God is somehow nullified by the new covenant. In fact, much of Romans is expressly dedicated to refuting such an antinomian idea.