Second mortgage lending has become a key component of the strategy of many subprime lenders, who are pitching piggyback combos as a way for borrowers to keep their payments low while eliminating the need for potentially costly mortgage insurance. There is no question that second mortgages are popular. According to the Inside Mortgage Finance MBS Database, second mortgages accounted for 15.7 percent of the subprime loans securitized during the first three months of this year.…