A good, irenic post at Ref21:
One of the ways in which modern advocates could strengthen the two kingdoms doctrine is by further emphasizing and clarifying its fundamentally eschatological character, particularly in light of the fact that the two kingdoms are often confused with two spheres into which life is to be divided. It may be that part of the problem is a conflation of the two kingdoms doctrine with Abraham Kuyper’s concept of sphere sovereignty. But Kuyper’s spheres denote different areas into which human life under Christ’s lordship are to be divided; they do not designate the eschatological distinction between this age and the age to come. As such, the concept of sphere sovereignty is a sociological concept that is consistent with but different from the two kingdoms doctrine. We confuse the two when we think of the two kingdoms as two spheres (because they denote two governments) but forget that they also denote two overlapping ages. As 1 Corinthians 7 and Ephesians 5-6 make clear, because Christians live between two ages, they cannot turn everything they do into the kingdom of God, but they are to do everything that they do in obedience to Christ’s lordship.