When, in April 2014, Bill Ackman and Michael Pearson came on stage together at the Equitable Center in midtown Manhattan to unveil their unorthodox bid to buy drug company Allergan, the two men couldn’t have appeared more different. Pearson, the CEO of Valeant Pharmaceuticals, was frumpy, overweight, and gruff. Hedge fund billionaire Ackman was, as always, impeccably groomed, trim, and silver-tongued. What brought them together was their joint love of big deals—and big profits. Ackman seemed in awe of Pearson’s dealmaking and his legendary thriftiness. In his remarks, Ackman pointed out admiringly that Pearson wouldn’t allow Valeant shareholders to be...