Billionaires Andreas Halvorsen and Bill Ackman are doubling down on Nvidia and Nike.Billionaire managing investors have the great responsibility of managing tremendous wealth for their clients, which necessitates the need for meticulous research into every investment.Andreas Halvorsen of Viking Global Investors and Bill Ackman of Pershing Square Holdings are both prominent billionaire fund managers who started positions in two members of the Dow Jones Industrial Average (^DJI -0.28%) in the second quarter a ...