In Paul and the Gift, Barclay wants to put a more helpful nuance on this. Rather than talk of simul justus et peccator, Barclay thinks it would be more Pauline to say simul mortuus et vivens, both dead and living. This captures, I think, a better sense of St. Paul's sense of the larger pattern of Scripture, and makes better sense of our calling to be Christians, little Christs.
Barclay follows recent scholarship in interpreting Romans 7 as an explication of Human life before conversion. This might be true, but there's no reason why we understand that to mean that our life according to the flesh doesn't follow after us. This is because we are dead in our sins, but alive in Christ. This is the Adam-Christ type applied to our lives, but restraining its use to a more limited form.
What I mean by this is that Adam's sin in the Garden was monumental, but it was not a fall from perfection to oblivion. His sin was highhanded rebellion against God, but it was not the pinnacle of corruption. In fact, Scripture testifies that it gets worse, as people manifest Adam's sin in fuller and more depraved ways. Scripture reveals development and maturity within static categories. Sin is a developing, or more accurately degrading, concept within the more static category of death. We may talk of dying as a process leading up to death, but we do not become more or less dead. Adam's sin in Eden ushered in the reign of death, where our sins climax. Adam remained a Man of Dust, and cut off from God's communion, was doomed. Living according to the Flesh thus means living according to an empty vessel, one gnawing with decay and rot. Adam's sin becomes more developed, growing in its pride and lust, becoming more Satanic in its bondage to death. This approach is a combination of Irenaean, Athanasian and Augustinian themes, which helpfully complement one another.
This is why St. Paul can say that Death is the final enemy, and yet focus so much on the sins of mankind. The wages of sin is death, and we are reaping such a harvest. However, Christ has brought eternal life, but through the cross. Thus, the salvation of God manifests in our world as Christ on the cross, or, as Chrysostom put it, trampling down death by death.
The Christian life is not one of radical schizophrenia or inhabiting a dual states, even if life in Christ reveals a dual movement. While we are still afflicted with sins, and will be throughout this side of glory, we now repurpose them. We put to death the deeds of the flesh, we mortify our sins, so that we may sow our bodies of corruption and reap a harvest of incorruption. The work of Christ radically repurpose our corrupted and evil state, where repentance turns the body of death into the vessel of God's redemption. The Christian life is thus not a contradiction, not a mere double inhabiting of Heavenly glory and Hellbound sludge. Rather, we live in the reign of Christ's cross, where putting to death the deeds of the flesh is a moment of victory and conquest.
Just as Christ reigned from the wood, so we are coheirs with Christ in Heavenly glory through putting our sins to death. Christ's ascension and reign is an anchor, and sure hope, not only of future glory, but the very flow of that glory. The Christian must follow his Lord through the waters of baptism, manifest in life through a life of repentance and putting sins to death. This is the path to glory. This is why horror of Romans 7 erupts suddenly into the joy of Romans 8. Our body of death is reconfigured by Christ into the mechanism of redemption.
It's not that Luther was wrong, but simul justus et peccator does not grasp the full depth of St. Paul's insistence that, indeed, while we are dead in the flesh, we are alive with Christ. While Christ was without sin, He became sin, and thus entered into our plight to deliver us. While death and resurrection are distinct, they are two movements in a single act. Hence, Union with Christ is the heart of salvation, encompassing even our sins for our good and God's glory.