China and the United States are not fierce economic competitors and tentative allies. In fact, in economic terms they are not two ‘countries’ at all, but rather a single, unified behemoth called ‘Chimerica.’ That, anyway, is the counterintuitive opinion of “Wall Street Journal” contributor and New York-based economist and global financial consultant Zachary Karabell. Karabell’s new book, “Superfusion: How China and America Became One Economy and Why the World’s Prosperity Depends on It,” outlines the emergence of a singular mega-economy that “is hiding in plain sight, unrecognized, unacknowledged, and unwanted” by “many millions whose lives are being reshaped by it.”