Actually, Marcion and Darby did teach something very similar.
Marcion taught that there were two Gods. Marcion taught that the God of the OT was a God of law, who was mean, and cruel. Marcion taught that the God of love was completely hidden until he was revealed in the person of Christ Jesus.
Darby (like Marcion) taught that God was not taught as a God of love in the OT, and revealed to mankind as a God of love until He appeared as Christ Jesus. Darby taught that before Christ Jesus, God revealed Himself to mankind in different ways.
Darby taught that if God revealed Himself one way to Israel, and another way Christ, then God also revealed Himself in different ways to Adam, Noah, Abraham, etc.
One of Darby's biggest errors was that Darby taught God was not a covenant God, but a God of Dispensations.
However, Darby's biggest error was that the promises to the Israeltes/Jews were fulfilled in Christ Jesus and His church.