You asked if the Christian has two natures. The answer is no; the sinful man is dead and buried. Unless you dredge it up from the grave, as many believes often do (See quote from Galatians 2 above).
I tend to agree with your position. According to Rom 6:6, Our 'old man', all that we are in Adam, was crucified with Christ. This means that we do not have a 'sinful nature'. Temptation to sin is external. The 'flesh' is not a sinful nature as such but our human nature under the control of sin.
The problem with the two nature idea is that it makes the individual a spectator and not the initiator of sin or obedience.
Also, the 'sinful nature' idea tends to make sin to be almost a physical 'thing' inside me, when sin is an attitude of self pleasing.