(FORTUNE Magazine) – CAN TWO HIGHLY accomplished, hard-driving managers share one job without a distracting and debilitating power struggle? Conventional wisdom has long answered that question with a resounding no. Yet given the ever-increasing complexity of running a global business these days, more and more companies seem willing to give high-level job sharing a try. IBM is one. In July, when John Joyce resigned as head of the company's $50-billion-a-year services group, Big Blue split his job between two senior vice presidents. Citigroup, meanwhile, has tapped two people to be co-chiefs of global consumer banking--replacing one executive, Marjorie Magner, who left the company earlier this month. Then there's Wall Street. Finance mavens love to argue about who might replace private-equity powerhouse KKR co-founders Henry Kravis and George Roberts when they eventually retire, but almost everyone agrees that the dynamic duo will most likely be succeeded by another twosome.

What does it take for two people to share a powerful position without driving each other--as well as everyone under them--crazy? Consider the experience of Jerry Greenberg and Stuart Moore, who have spent the past 15 years as co-CEOs of Sapient, a consulting firm headquartered in Cambridge, Mass., that numbers Procter & Gamble, Wal-Mart, Fidelity, Motorola, and the U.S. Marines among its clients. At first glance, Greenberg and Moore might seem an odd couple. Moore is a Republican who lives in Boston; Greenberg is a Democrat based in Los Angeles. A few lessons that this pair learned--sometimes the hard way--may offer guidance for anyone sharing a job at any level of an organization.

● Have distinct roles. Moore is Mr. Inside, heavily involved in the day-to-day running of the company; Greenberg deals mostly with the outside stuff, like marketing and investor relations. But such division of labor only works with two highly competent people. "I think what has really helped us is that each of us believes that either of us could run the company alone," says Greenberg. "Rather than seeing ourselves as two halves of a whole, we think we're each individually capable of doing the job."

● Admit you can't do everything. Sounds obvious, but Greenberg has seen plenty of power-sharing arrangements torpedoed "because each person secretly, or even not so secretly, thinks, I'm really the only one who should be running this." Moore agrees: "That kind of thinking just leads to a lot of second-guessing, and that will just drag you down." The pair often spend hours discussing a single aspect of the business, until they come to what Moore calls a "profound accord, which sends a powerful message to the rest of the company. Otherwise you just generate noise."

● Hire a referee. Early on, Greenberg and Moore set up a system that anyone sharing a job might want to try: They chose three colleagues who would act as referees and tie-breakers if they ever ran into a conflict of opinion that they couldn't resolve on their own. "So far we've never had to use that process," says Moore, "but we haven't dismantled it either." Sometimes it's a matter of picking your battles, he adds: "Usually, when we see things differently, one of us feels more passionate about whatever it is than the other one does." In those deadlocks, "the more passionate one wins, but that doesn't mean 'whoever screams the loudest,'" explains Greenberg. "We try to get to the best answer, and if one of us has a clearer sense of what that is, then that's how we go."

● Don't confuse the troops. If you ask people to report to more than one boss at a time, it's bound to be a disaster. "It took us three or four years, at the beginning, to figure this out," says Greenberg. "Nobody reports to both of us, ever." For instance, Sapient's chief marketing officer normally reports to him, but if Moore borrows her talents for a short-term assignment, she answers only to Moore for the duration. "Especially in a company that's growing fast, or is rapidly evolving, you want to eliminate any potential for chaos and confusion," Greenberg says. "It helps a lot if people know exactly who their boss is."

Greenberg still seems a bit surprised to find himself sharing the CEO post. "When I was a student at Harvard, they drummed into us that this kind of structure never, ever works," he recalls. Silviya Svejenova, who teaches strategy at the ESADE graduate business school in Barcelona, begs to differ. She is co-author of the forthcoming book Sharing Executive Power: Roles and Relationships at the Top (Cambridge University Press, $34.99, November 2005). "A common misconception is that sharing a high-level job diminishes each manager's power. But our research shows just the opposite," Svejenova says. "In cases where the right people are paired, each person's power is increased by the alliance. The whole becomes more than the sum of its parts."

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