Piggyback mortgages remain a major challenge for mortgage insurers, which have seen the com-bination products chip away at their market share the last few years as originators pitched 80-10-10s, 80-20s and other blends as a way to keep loan payments low. But the tide may be changing. Last month, Standard & Poor’s made a change to its rating criteria that places higher credit enhancement levels on first mortgages that are covered by a second...