Prof. Le Dinh Luong, president of the Genetic Association of Vietnam, says DNA testing at his Hanoi lab confirmed the twins have different fathers, the first such case that he knows of in Vietnam, with only seven such cases reported in the world as of 2011.
Luong declined to give details because of confidentiality with his client.
Online newspaper Dan Tri reported that a 34-year-old man from northern Hoa Binh province had DNA testing after being pressured by his family because the twins did not look alike. One has thick wavy hair while the other has thin and straight hair.
To rule out a hospital mix-up, DNA testing of the mother showed that she was the mother of both children, the report said.
The twins who are now two years old were born the same day and have same sex, it said.
Luong said bi-paternal twins could happen if two eggs from the same mother were fertilized by sperm from two different men within one to seven days apart in an ovulation period.