Fannie Mae and Freddie Mac quietly became major investors in the subprime mortgage market last year, piling up a combined $49.59 billion in subprime mortgage-backed securities business, according to a new analysis by Inside MBS & ABS, an affiliated newsletter. Both GSEs began their involvement in the subprime MBS market by providing securities guarantees on deals issued by major lenders, and in some cases issuing the securities themselves. Freddie Mac did $11.11 billion of