Doctors state that the mothers egg was fertilized by two of the fathers sperm before dividing. Doctor Gabbett states that in this case, the fertilized egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating twins. Some of the cells have the chromosomes from the first sperm, while the remaining cells contain chromosomes from the second sperm, which results in the twins to share only a portion, rather than 100% of the same paternal DNA.
This is the second case which has not taken place since 2007 in United States. Mostly fraternal twins are highly possible for boy and a girl. The doctors are still studying this case, and are hoping for the mother to have a successful pregnancy so this could be further documented. I believe that with this new fertilization, doctors are able to identify, the how and the why of this pregnancy and would be able to procreate this method, since artificial insemination is now highly popular.