By Mark Lamport-Stokes ATLANTA, Sept 12 (Reuters) - Seven years after being fierce rivals at the Walker Cup, Billy Horschel and Rory McIlroy will play together in Saturday's third round of the Tour Championship where playoff honours and a staggering $10 million bonus are on the line. American Horschel, the game's hottest player over the past three weeks, holds a two-shot lead in the PGA Tour's season-ending event but Northern Irishman McIlroy, the world number one, is close behind in a tie for second. While Horschel is bidding for a second consecutive triumph on the U.S. Both players have already achieved a great deal in professional golf and have become good friends since they locked horns as amateurs in the 2007 Walker Cup at Royal County Down in Northern Ireland. More...