You misunderstand the scripture.In every real believer, there is one, divine, nature.
As it is written in 2 Cor 5:17..."Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new."
My old nature passed away, and I was given a new one along with my new life in Christ.
And, as it is written in Gal 5:24..."And they that are Christ's have crucified the flesh with the affections and lusts."
Having no more lusts to please, enables a life focused entirely on the Spirit.
Galatians 5:17
For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
A lost person does not have the Spirit lusting against the flesh and does not subsequently find trouble doing those things that are right he wants to do.