Fannie Mae and Freddie Mac in July posted a combined increase in the volume of single-family mortgages they securitized, according to a new Inside The GSEs analysis.Fannie and Freddie issued $57.9 billion in single-family mortgage-backed securities in July, a 12.3 percent increase from June. However, July’s MBS issuance was down 59.0 percent on a year-to-date basis.