Feeding art-world anticipation of one of the biggest auction seasons in history, the entertainment mogul David Geffen has sold in private transactions two postwar paintings by Jasper Johns and Willem de Kooning for a total of $143.5 million. The buyers, Kenneth C. Griffin and Steven A. Cohen, are among today’s most successful hedge fund billionaires and are both building high-profile art collections. Mr. Griffin, managing director and chief executive of the Chicago-based Citadel Investment Group, and his wife, Anne, bought “False Start,” a seminal 1959 work by Mr. Johns, for $80 million. Mr. Cohen, the founder and manager of SAC...