Answer: Tom Scott and Andrew Carnegie
Explanation: Carnegie worked for Thomas Scott, one of Vanderbilt's biggest competitors, as a personal assistant. Thomas Scott liked Carnegie and taught him much of his railroad knowledge. In 1865 Carnegie left the railroad business and deepened into the steel business - years later there is the Carnegie Steel Corporation, today US Steel.
Carnegie was a business partner of Thomas Scott, who was perhaps Rockefeller's enemy for trying to influence his business. As such, Rockefeller destroyed Scott's company and gained a foe - Carnegie.