Both Fannie Mae and Freddie Mac retained their dominant shares of mortgage-backed securities with a bit of a boost during the third quarter of 2012, according to an Inside The GSEs analysis.The two GSEs issued a combined $335.4 billion MBS during the third quarter, compared to $273.9 billion during the previous quarter. Fannie and Freddie saw an ample 54.7 percent increase in MBS issuance during the first nine months of 2012 compared to the same period a year earlier.