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Paul takes away any notion of there being 'two systems in Christ', if I can put it that way. He says we are "all one in Christ". But...in case there are those (for indeed there are) who fear that the promises made to Abraham and the nation of Israel are therefore not being fulfilled by God in this manner, Paul goes on to say:
But it is not as though the word of God has failed. For not all who are descended from Israel belong to Israel, and not all are children of Abraham because they are his offspring, but “Through Isaac shall your offspring be named.” This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring. -Romans 9:6–8
So...not all Jews are "Israel", right? It's the "children of the promise" who are "counted as offspring". Who are the "children of promise"?
And if you are Christ's, then you are Abraham's offspring, heirs according to promise. -Galatians 3:29
That would be us...the Church. But of course, according to Paul, the Church has only been grafted onto the "root":
But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree, do not be arrogant toward the branches. If you are, remember it is not you who support the root, but the root that supports you. -Romans 11:17–18
So, you can see why there cannot be a 'separate' plan, or a 'difference in application' of the salvational plan according to scripture. There is Christ, and there are those who are growing from him, either naturally, or by being grafted there. Jews who believe in Christ are grafted in, just as the Gentiles were, and become Christians, members of the church.
But nowhere in scripture does Christ, or Paul, give us any sort of leave to say that any of these Jews who come to believe in Christ, become anything other then members of the Church, fellow believers like you and I, children of promise.