As a believer, our essential nature is "spirit". That spirit is either alive "in Christ" or dead "in Adam". To have 2 natures would mean a person possesses 2 spirits...quite an impossibility since your essential essence is spirit...in other words, when you die your spirit (singular) is separated from the body ((James 2:26 NASB) "For just as the body without the spirit is dead...."). You are a spirit being who lives on this earth in an earth suit we call a body...that spirit has life in Christ or is separated from life in Christ (i.e. death).
The capacity to sin still remains in the body (Romans 7 - this does not make the body evil, it simply means this body and our mind is susceptible to sin until we receive our heavenly bodies) and satan will use the old patterns of believing and behaving to deceive us into making wrong choices.
(2 Cor 6:16 NASB) Or what agreement has the temple of God with idols? For we are the temple of the living God; just as God said, "I WILL DWELL IN THEM AND WALK AMONG THEM; AND I WILL BE THEIR GOD, AND THEY SHALL BE MY PEOPLE.
The believer has the Lord Jesus Christ living on the inside (i.e. in their spirit). Since the Lord can not be joined to evil, it is not possible for the nature of the believer to include evil (1 Cor 6:17 NASB But the one who joins himself to the Lord is one spirit with Him. (i.e. we are baptized in the spirit. We are not God, but are joined together with Him)).