Together, Fannie Mae and Freddie Mac in September posted a combined increase in the volume of single-family mortgages securitized, according to a new Inside The GSEs analysis. Fannie and Freddie issued $64.1 billion in single-family mortgage-backed securities in September, a 4.9 percent increase from August. However, September’s MBS issuance was down 56.7 percent on a year-to-date basis.