Riskier single-family mortgages securitized by Fannie Mae and Freddie Mac in recent years continued to drag down earnings in the third quarter of 2010 and force the GSEs to go deeper in debt to the federal government. The two companies lost a combined $8.6 billion on their single-family MBS guarantee programs during the third quarter, a modest improvement over the $9.6 billion they lost during the previous quarter, according to the Federal Housing Finance Agency’s latest…