File photos of Carl Icahn and William Ackman

A combination file photo shows investor Carl Icahn (L) speaking at the Wall Street Deals & Deal Makers conference in New York, June 27, 2007 and William Ackman, CEO of Pershing Square Management LP at AGM of CP Rail in Calgary, Alberta, May 17, 2012. The two biggest hedge fund activist investors in the world, Carl Icahn and Bill Ackman squared off on CNBC Friday during a lunchtime interview exchanging verbal attacked which was watched closely on the New York Stock Exchange floor and Twitter. REUTERS/Staff