This could be his biggest multimillion-dollar payout yet. Flamboyant philanthropic billionaire Stewart Rahr and his wife, Carol, are getting divorced after 43 years, Page Six has exclusively learned. We’re told that Stewie Rah Rah, the self-styled ‘king of all fun,’ and Carol have been living increasingly separate lives in recent years, even though they still live together in a huge Trump Park Avenue apartment and a $43 million house in the Hamptons. Carol served Stewart with divorce papers late last week as he continues to give large amounts of his $1.8 billion fortune to charity. In what could be one of New York’s costliest divorces of all time, Carol — who has two adult children with Stewart — has hired Robert Stephan Cohen, described as one of the most powerful matrimonial lawyers in the country, and the “pit bull” of all divorce lawyers. He has represented the likes of Christie Brinkley in her divorce with Billy Joel, Ellen Barkin and Uma Thurman, and is famous for playing tough. But a source told us, “The divorce is expected to be amicable and friendly — it’s not going to be ugly. But it will end in an enormous settlement, because Carol has been at his side as he built his company, and stuck by Stewart through thick and thin.” Stewart, a self-made, Brooklyn-born billionaire, sold his pharmaceutical company Kinray for $1.3 billion in 2010, has since devoted himself to giving his money to charity. The yellow-Ray-Ban-wearing billionaire says he has given away $25 million in two years, most recently giving $7 million to the Prostate Cancer Foundation for research. Through his philanthropic work he has developed friendships with power brokers including Bill Clinton, Michael Milken, Alicia Keys and Andrea Bocelli, who both recently performed at Stewart’s lavish annual Hamptons charity bash. Ben Brafman, attorney for Stewart Rahr, confirmed that Stewart and his wife, Carol, were ending their marriage. Brafman said, “Mr Rahr intends to act honorably and fairly. He is hopeful and confident that the parties will come to an amicable agreement and that despite ending their marriage, will remain friends in the years ahead.” Robert S. Cohen could not be reached last night.