How do those who believe in a dual system (Old & New Covenant simultaneously operative) pre-70 AD understand "one Lord, one faith, one baptism"?

Some hold that the Old & New Covenants were simultaneously operative and efficacious between the death of Christ and the destruction of Jerusalem in AD 70. How do those who hold this view understand Paul's declaration that there is "one Lord, one faith, one baptism"? (Ephesians 4:5) Would the simultaneous availability of both covenants contradict Paul's statement? *** Inspired by discussion on this post I am assuming here that Paul is the author of Ephesians and that therefore Ephesians was written pre-70. For a more thorough exposition of this view see chapter 3 here . I understand that some may answer this question by indicating that they do not accept the authenticity of Ephesians--this would resolve the apparent contradiction but would not directly address my specific question.