Where do the genealogies of Mary and Joseph split from each other?
The genealogies of Mary and Joseph split at Solomon. From David, the family tree branches at Solomon: one line descends to Mary, the other to Joseph. Though Joseph was not Christ's physical father, he was the legal father, and the royal line through David and Solomon establishes Jesus as the rightful heir to David's throne.
Les explains that Matthew's genealogy in chapter 1 is given to prove Jesus is the rightful heir to the throne of David, so it traces the royal legal line down to Joseph, the legal father of Christ. It stops at Abraham because that is as far as this particular genealogy concerns itself. Luke chapter 3, by contrast, gives Mary's genealogy and goes all the way back to Adam, ending with Adam through Seth and Enos. Joseph was not the physical father of Christ, but he was the legal father. Coming up from Abraham to David, then to Solomon, the family tree branches: one line comes down to form Mary's genealogy, and the other line forms Joseph's genealogy. So the split occurs at Solomon.