For years BHP and Rio Tinto, the world’s two most valuable miners, moved in lockstep. During the 2000s the twin Anglo-Australian giants rose on the back of China’s demand for commodities, particularly iron ore. In 2007 they even explored a merger (regulators rebuffed the idea). Then, when the commodity supercycle crashed in 2015, both landed in investors’ bad books, and were forced to shed assets and pay down their debts. Now, as the…