Fannie Mae and Freddie Mac may be eyeing different tacks as far as buying MBS and building their retained portfolios in the face of dwindling product supplies and ongoing strong competition. Freddie Mac bought $7.1 billion more MBS than Fannie in July. Freddie purchased a total of $28.7 billion in mortgage securities during that month, while Fannie acquired $21.6 billion. And Freddie’s portfolio balance…