Actually, some sources say Nestorianism claims two persons and some say two separate natures. I unfortunately don't know any Greek (yet), so I can't follow the controversy in original sources. But, using "natures," Nestorianism claimed that Mary was the mother only of the human nature and -- as a corollary -- that only the human nature suffered and died on the Cross, which leaves the theology of the Atonement rather in tatters . . .
The two natures of Christ are not divisible. Dividing them is heresy.
Besides, since is His mother, He is her Son. She is mother of a Son, not a nature.