Although Augustine professed to have denounced his former beliefs in the doctrines of Mani and wrote copious refutations of his heresies, the profound dualism espoused by his former teacher did not depart him. This became troublesome for Augustine, not only in the theory of Traducianism noted above, but in his conception of the Incarnation itself. Augustine could not conceive that the Spirit of Christ could actually join itself to the corrupt nature of the flesh. As he says, “For as the soul makes use of the body in a single person to form a man, so God makes use of a man in a single person to form Christ. In the former person, there is a mingling of soul and body; in the latter, a mingling of God and man… when the Word of God unites to the soul which has a body, taking thereby both soul and body at once… it ought to be easier to intermingle two incorporeal things rather than one incorporeal and the other corporeal.”48 So, in Augustine’s view, the soul was the middle man which enabled Jesus to be united in body and Spirit without the one having to be joined to the other (positively Gnostic!).
There seems to be a slight confusion of Manichaeism with Platonism. Dualism in Manichaeism is one in which eternal forces of light eternally battle eternal forces of dark. This is not Augustine’s belief, and the dualism in Augustine (the divide of the spiritual and the physical) was a Platonistic idea. Platonism held that there were three hypostasises: The realm of the One, an eternally unchanging perfection that cannot be related to anything else. The realm of the Intellect, a near perfection state in which the changeable is suppressed. And the realm of the Soul, which is made up of changeable mater.
In Platonism, the realm of the Soul is populated because of corruption of the Intellect. In this way, both the Intellect and especially the One cannot be associated with the realm of the Soul. Augustine takes an idea from Plotinus that bodies have spirit elements within them. But bodies, and all changeable matter, need to be ultimately discarded in favor of the unchanging.
Augustine’s ideas of the incarnation more accurately reflect Platonistic sensibilities than those of Manichaeism.
God is Open 
From my studies on Augustine, it seems like Augustine was still a philosopher and so he mixed different opinions, made up his own stuff,… When forming “orthodox” doctrines, he seemed to have been mostly influenced by Platonism. See my upcoming dissertation! :-)