Jesus and the overlapping ages
The notion of two overlapping ages comes from Jesus himself - whose life was lived in the conflict which comes from the overlap of two kingdoms.
In his resurrection from the dead, Jesus gave an unexpected first instalment of (and the means to) a general resurrection, the one having occurred, the other being yet to occur.
In his gift of the Spirit, Jesus gave only an earnest of what was to come (2 Corinthians 1:22; Ephesians 1:13-14), the one having occurred, the other being yet to occur.
With the coming of Jesus, judgment itself was divided into two parts: there was one sense in which judgment had come now with the coming of Christ and the gift of the Spirit - Matthew 3:11-12; Acts 2:17-21; there was another sense in which judgment would only fully be expressed at the final judgment - Revelation 20:11ff
In his very person, Jesus introduced the totally unexpected idea that the "age to come" would exist side-by-side with "this evil age", until the final judgment.
Israel expected these things to occur simultaneously with her national vindication. Jesus split them apart, and destroyed geographic Israel’s national identity.
Hebrews 6:5 speaks of those "who have shared the Holy Spirit, who have tasted the goodness of God and the powers of the coming age." The powers of the coming age were the workings of the Holy Spirit which would be fully experienced in an age yet to come - and which is still yet to come (unless you think we have experienced greater powers than these in church history since the time of the apostles).
The very notion of the kingdom rests on the assumption of one kingdom, the kingdom of God, breaking in on "this evil age" - as Jesus everywhere modelled it in his ministry, and as it continued to be modelled by the apostles. That is still the case today. There is still a conflict between two kingdoms, nowhere better demonstrated than in the conflict which frequently takes place when one person is transferring his/her allegiance from one kingdom to the other, or when there is any significant advance of God’s kingdom anywhere in the world.
You don’t need German theologians to come up with this idea: it’s there in the person of Jesus himself, in his gift of the Spirit, in his inauguration of the kingdom, in his bringing of judgment, in the way he challenged and modified the eschatological expectations of Israel, in the experience of the church in the apostolic era, and in every era since then, continuing to the present day.