George Soros and Steve Cohen are expanding their stake in an Argentine tech start-up that aims to transform how people in South America’s second-largest economy save and spend. Ualá, which launched its mobile banking app in October with backing from the billionaire investors, raised $10 million in a funding round of Series A stock led by Soros that closed last week, according to Soros Fund Management. Cohen’s Point72 Ventures and Jefferies Group LLC also participated. In a country where less than half the population has a bank account, Ualá offers tech-savvy Argentines a low-cost banking alternative via their smartphones.