Imagine a world in which two asset managers call the shots, in which their wealth exceeds current U.S. GDP and where almost every hedge fund, government and retiree is a customer. It’s closer than you think. BlackRock Inc. and Vanguard Group  — already the world’s largest money managers — are less than a decade from managing a total of $20 trillion, according to Bloomberg News calculations. Amassing that sum will likely upend the asset management industry, intensify their ownership of the largest U.S. companies and test the twin pillars of market efficiency and corporate governance.