Both Nestorius and Pelagius believed in a composite subject Christology that the person of Christ is a divine-humane person, the Logos and His tabernacle. Their Christology were condemned at Ephesus (431) and later repeated at Constantinople (553) where both Ss. Cyril of Alexandria and Augustine of Hippo are declared as the Doctor of the Church. Nestorius teaches that the Logos is united with a perfect man. While Pelagius teaches that the perfect humanity of Christ is united to the Logos. So that both teach the Logos unites Himself with a perfect tabernacle as a composite subject.
St. John Cassian under a request by Pope St. Leo the Great which at that time was an archdeacon wrote seven volume treatises to combat Nestorianism by connecting it with Pelagianism.
[T]he error of Pelagius ... that in saying that Jesus Christ lived as a mere man without any stain of sin, they actually went so far as to declare that men could also be without sin if they [chose].
John Cassian, On the Incarnation against Nestorius, 1:3, in The Works of John Cassian, trans. with preface Edgar C. S. Gibson, in The Nicene and Post-Nicene Fathers, 2nd ser., vol. 11, ed. Philip Schaff.
In similar fashion from a different perspective Marius Mercator accuses Theodore of Mopsuestia of being the father of the Pelagian heresy for teaching,
[T]hat the progenitors of the human race, Adam and Eve, having been created mortal by God, did not wound any of their descendants when they strayed by their transgression, but harmed only themselves; that they made themselves guilty of the command before God, but absolutely no one else.
Marius Mercator, Commonitorium, Patrologia Latina, vol. 48, 110D-111A.
Both John Cassian and Marius Mercator linked Nestorianism with Pelagianism. Richard Norris in his study Manhood and Christ observed that Antiochene Christology is aptly enough expressed in the dictum that, “The Nestorian Christ is a fit Savior for the Pelagian man.”1
So how are their Christologies differentiated from one another?
1 Richard Norris, Manhood and Christ, Oxford: Clarendon Press, 1963, p. 246.