They were young lovers in London, working as junior investment bankers at Moelis & Co and Centerview Partners. They had affectionate nicknames for each other: Pops and Popsy.

From that unlikely perch, federal prosecutors allege, Darina Windsor and Benjamin Taylor orchestrated a years-long global insider trading ring. They stole confidential bank documents about impending corporate transactions and shared them with middlemen who handed the information to traders—yielding tens of millions of dollars in allegedly illegal profits, prosecutors said.