Fannie Mae and Freddie Mac, the housing-finance companies supported by US taxpayers, should take advantage of demand for government-backed mortgage debt and sell their holdings, according to Pacific Investment Management Co. (Pimco). “Since the government’s going to want to unwind them at some point anyway, why not do it at the best levels ever?” said Scott Simon, the mortgage-bond head at Pimco, manager of the world’s biggest fixed- income fund.