Jesus summarises the two groups as two great commandments: love God with all that you are and have, and love your neighbour just the way you love yourself. So ten become two. Or perhaps better, the two second-order commandments that stand behind the ten third-order commandments are now revealed.
Second-order? Yes - because in the person of Jesus, those two are one. Think about the incarnation. Jesus is God. I therefore owe him all of the obedience that is demanded by the first 'table' of the commandments, and by the first of the great commandments. He is the God beside whom there can be no others, before whom there must be no idols, against whom there must be no blasphemy, and in whom there must be sabbath rest. But Jesus is also man. I therefore owe him all the obedience of the second table - he is alongside me as the neighbour I must not wrong.
Incidentally, this is confirmed by NT ethics. This especially springs to mind, from Galatians 6:10: "So then, as we have opportunity, let us do good to everyone, and especially to those who are of the household of faith." Why is there a distinction here, between 'everyone' and 'the household of faith'? Is Paul limiting the people I should do good to? Clearly not. But he is highlighting that here, in the household of faith, Christ as neighbour is most clearly seen, because Christ is in my neighbour and vice versa. Therefore, my ethical response of neighbour-love within the church is especially appopriate.
So the first-order commandment, which stands behind the two-fold great commandments and the decalogue, is Jesus. All the commandments are meant to lead me to him. I am meant to be bound to him in love and trust, and in being bound, to be free - here I can be the one I was made to be.
Does this help with our view of the Mosaic law in Romans 9-10? Israel chased the law, and the righteousness it promised, but did not see Christ as the fulfilment of the law and its ultimate end. They were left, then, without faith, pursuing the outward form of the law without Christ, who is the beating heart of the law and of all believing ethics.