(FORTUNE Magazine) – Talk about corporate love affairs. Ask McDonald's CEO Mike Quinlan about his company's relationship with Coca-Cola Co., and his eyes light up. "Just wonderful," he gushes. "They are our partner." Echoes Don Keough, Coke's former No. 2 guy and now a McDonald's board member: "It's an enormously important strategic alliance. McDonald's is a hallmark customer." No kidding. Not only is McDonald's the largest fountain-sales customer of Coke, but the two global giants also share a common No. 1 enemy: PepsiCo, which in addition to making Pepsi Cola owns fast-food rivals Pizza Hut, Taco Bell, and KFC. Like any successful affair of the heart, the two companies enjoy getting together for those special occasions. Every 18 months or so, the top 100 executives from each company meet for a weekend at some locale like Palm Springs or Palm Beach, and they don't scrimp on the speaking fees. In past years, for example, they have given the podium over to Ronald Reagan, Margaret Thatcher, and Henry Kissinger. The next gathering will be held this February in Phoenix, with an undisclosed heavy hitter scheduled to speak. "The focus of these meetings is on the future of the global marketplace," says Keough. "The companies have much common ground there." For example, the two companies worked together to open up the Russian market earlier this decade. The relationship has a long history, going back some 40 years to McDonald's infancy. The payoff for Coke has been tremendous. Coca-Cola products are served in all 14,500 or so of McDonald's stores and account for about 5% of its U.S. volume. Not a single restaurant serves Pepsi -- although there is no actual company edict that prohibits it. The heresy of that act would be unthinkable to a McDonald's franchisee. Those of us who aren't invited to these big powwows can certainly speculate that they're a great opportunity to get out on the links. Or plot the total destruction of PepsiCo? Not so, insists Keough. "We really don't get into that," he says, leaving the impression that McDonald's and Coke are too busy looking deeply into each other's eyes and swooning to bother with the competition. Not to mention carving up the world.