Fannie Mae and Freddie Mac both saw substantial increases in single-family volume during the second quarter, aided in part by nonbank sellers scouring for refinance business. But Fannie enjoyed bigger gains, while Freddie’s share of the two-horse GSE market slipped to 39.5 percent. Freddie’s share has hovered above 40 percent for the past two years, and it was 40.2 percent for the first six months of the year, but the GSE has to prop up its share by charging lower guarantee fees and through other means. In 2016, Freddie has been getting a smaller share of some sellers’ business than it got in the first half of last year. [includes two data charts]