The multibillion-dollar trading firm said its billionaire co-chief executives, John Overdeck and David Siegel, would step down. : The multibillion-dollar trading firm said its billionaire co-chief executives, John Overdeck and David Siegel, would step down. Two Sigma, one of the world’s largest hedge funds, said on Wednesday that its billionaire co-founders, John Overdeck and David Siegel, would step down as co-chief executives. Mr. Overdeck and Mr. Siegel, who founded the multibillion-dollar firm in 2001, have been engaged in a long, bitter feud that has spilled into public view.