Ushering a guest into his office on Manhattan's West 57th Street, Prince Egon von Furstenberg, 35, gestures toward a window. "Diane and I can see each other from our offices," he says, pointing across town to a tiny speck, which is the window of her Fifth Avenue showroom. "We stick our heads out, wave and send signals." He laughs self-consciously, for he is talking about Diane von Furstenberg, the fashion designer who is both his estranged wife and his best friend. It was she—not he—who made the name von Furstenberg famous.

In the eight years since they separated, Egon has worked at reclaiming that name. He is now the impresario of a $40 million-a-year men's suit business whose franchised line sells in stores like Marshall Field and Macy's. He has also written top-selling books on men's grooming (The Power Look) and home furnishings (The Power Look in Home Decorating for Men), and he recently became an interior designer. Having decided that "a suit is a suit, and there's not much you can do with it," he teamed up with Manhattan designer Marc Janecki to create a new firm. The commissions are rolling in already. Among them: a model apartment in Fifth Avenue's new Trump Tower, where Sophia Loren and Johnny Carson have already rented.

Von Furstenberg is planning a still bigger move—a second marriage next March. When he told Diane he wanted a divorce, "she cried," he says. Egon will marry Lynn Marshall, co-owner of a flower shop and his somewhat steady since 1979. Marshall, 29, a native of Mississippi, is not particularly career-minded, which is fine with Egon. "After marrying one woman with so much punch," he says, "I'm happy to have someone who settles for needlepoint."

Such a marriage will be more in keeping with Egon's aristocratic background. He is the son of Prince Tassilo von Furstenberg (the title originated in 11th-century Germany) and Clara Agnelli, sister of Fiat king Gianni Agnelli; Egon also is a distant cousin of Princess Caroline of Monaco and Sweden's King Carl XVI Gustaf.

Born in Switzerland, Egon was raised in a Venetian palazzo surrounded by a staff of 21. He had his own wet nurse, nanny and tutor, and attended the exclusive Le Rosey School in Geneva, alma mater of King Farouk and the Shah of Iran. "I was out of touch with reality," admits Egon, who at 18 sought to compensate for his pampered life by teaching for 14 months in a peace mission in Burundi. In 1965, while studying economics at the University of Geneva, he met fellow student Diane Halfin, the 18-year-old daughter of a wealthy German businessman. Their relationship blossomed—Egon says she became pregnant the day they got engaged. They married three months later, in July 1969, and settled in New York. There, shortly after the birth of her son, Diane started her dress business. Egon worked as a banker. The von Furstenbergs were lionized for their trendy life-style and frank discussion of sexual dalliances with others. "We were a bit of the social history of the time," shrugs Egon. "We said what most people then were afraid to admit about marriage—that it is boring."

Theirs effectively ended in 1973, but as Egon puts it, "Our friendship is forever." They share decision making about their children, Alexandre, 11, and Tatiana, 10. They see each other every day and often double-date. "Diane calls me in the middle of the night and asks, 'Do you love me?' I tell her, 'Of course I do. Now go to sleep.' "

Ironically, while Egon encouraged Diane in her dress business—and helped select some of her earliest designs—he lacked the confidence to pursue his own fashion career. It was not until they separated that Diane said to him, "You always push others. Why don't you push yourself?"

While doing so, Egon never lost his appetite for after-hours pleasure. He was invariably on hand for parties at Studio 54 and Doubles, usually with someone new. "I can't stay with the same person for long," he explains. "I get bored sexually." That attitude led him to experiment with a male partner. "I'm very pleased," he insists. "I have no regrets." As for Egon's marriage plans, Diane is dubious. "I haven't given my okay on Lynn yet," she smiles.

Whatever happens, her relationship with Egon is expected to remain solid—"I wouldn't dream of trying to change that," stresses fiancée Lynn. Her husband-to-be will continue to shuttle around the globe six months out of the year on business. Home is seven rooms on Park Avenue—too small, he complains, to accommodate more than 100 for dinner. Egon insists his wild days are over. "I took every drug possible and went out a hell of a lot," he says. "But what interests me now is my career. I didn't used to like myself. Now I do."