Airtel Payments Bank has found a way to make a virtue out of necessity.In September, it launched a campaign to get people to open their “second” bank accounts with it. The eight-year-old payments bank is the largest by revenue and has 51 million customers, but that’s still about half of what a large lender like HDFC Bank has. So to bring in more users, it wants other banks’ users to also use the payments bank, not because it’s any more convenient, cheap, or fast, but for safety.The country saw payments frau ...