"I used to say I was wearing two hats," says Beverly Sills. "Now I'm beginning to think I'm wearing two heads."

For a 51-year-old international star who is going into eclipse, America's favorite soprano seems remarkably calm and contented as she warms up for her last few singing dates. The eclipse is voluntary -- long-planned and eagerly awaited. Sills first began to think of a job change about 10 years ago, before her singing career had peaked, when someone asked about her unfulfilled ambitions and she said, "I'd like to run an opera company."

Now, she is running The New York City Opera, and tonight she will be the star singer in her own company's "Barber of Seville" at Wolf Trap. Before a final, gala farewell in "Fledermaus" in New York next October, she will sing twice at Wolf Rap, twice in New Orleans and six times in San Diego. And after than, she will be Beverly Sills, general director of New York City Opera. Period.

The idea of retirement as a singer did not come through a single traumatic incident but "an accumulation of a lot of little things," she says. First, there was an illness in the mid-70s."With me, it began as a shortness of breath. I began noticing it after an operation for cancer, and I thought that I could wait a while and my breath would come back. It didn't.

"Then I began to notice that I had to think about my high notes -- to plan them. I used to be able to whip out a whole string of high E-flats without a care in the world. I never had to think about my voice, it was always there -- not always perfect, but there."

When she decided to retire, her vocal coach told her she was crazy -- she could stretch the career another 10 years by doing easy repertorie. But she says, "I didn't want to compromise and I didn't want to give less than my best. I knew that at 48 I couldn't do things as well as I could at 42. When you become a -- please pardon the expression -- superstar, there will always be people around to tell you how good you are. And if the critics don't notice your problems, you are happy to get away with it. But that doesn't make the performance any better."

Sills takes the transition lightly, with a reminder of new responsibilities:

"My whole career -- I had such fun with it. I had a ball. Now instead of two vocal cords, I have to worry about 300." One of her most urgent new assignments will be to find and develop successors to Beverly Sills. She has several candidates in mind already, and she talks about them with an open enthusiasm that contains no hint of jealousy or even nostalgia. Nothing of the traditional overtones that have come to be associated (for substantial reasons), with the title of "prima donna."

It is an amazing change of pace for the woman who was born as Belle Silverman a half-century ago in Brooklyn but knew by the time she was 7 that she wanted to be a singer called Beverly Sills. Occasionally, a performing artist can make the transition to top management in a major opera company. Conductor James Levine has done it at the Met, and baritone George London was doing it at The Washington Opera before health problems cut short his second career. But a soprano? A coloratura soprano? Unthinkable!

But not for the red-headed superstar who began singing in 1932 on a radio program called "Uncle Bob's Rainbow House," went professional by singing radio commercials, had memorized dozens of arias when she was still in grammar school, sang to rabidly enthusiastic capacity audiences at every major opera house in the world (though deplorably late at the Metropolitan) and adopted as a motto in recent years, "I've already done that." Other things she had done included marrying journalist Peter Greenough in 1956, having two children of her own and adopting three others.

One thing she hadn't already done by 1978, when Julius Rudel began talking about making her a co-director at the City Opera, was run an opera company. But it had been on her mind as early as 1970.

"When I began thinking about it, I was thinking of Sarah and me," says Sills. "Sarah" is Sarah Caldwell -- founder, powerhouse, director and conductor of the Opera Company of Boston, who was giving star roles to Sills long before she had made her triumphs at LaScala, the Met or even the City Opera, which later became her main showcase.

"I thought I could relieve Sarah of some of the administrative and fund-raising routine, which does not much interest her and is not her most outstanding talent," Sills says. "But when the subject first came up, my voice was in its prime and I thought I should wait a while."

Rudel's proposition three years ago was better-timed. When he first raised the possibility (in a restaurant in California, she recalls), Sills had already done everything at least once and she was beginning to get messages from her voice. She would probably be retiring on about the same schedule, she thinks, even if she did not have an exciting new career waiting for her.

"I am my own severest critic," she says. "I don't have to read the reviews to know what went right or what went wrong. When I found that I did not meet my own standards, I was devastated. But then I thought, 'I have known this voice for a long time, and it has served me well. If it wants to go to bed, I should let it go to bed with dignity.'"

Once the resolution to retire was made, there were still problems. A figure like Sills has a momentum like alocomotive. "You cannot bring this kind of career to an imediate stop," she says. When she agreed to accept Rudel's offer, she says, "I was booked five years in advance -- up to 1983 -- literally millions of dollars in engagements." Winding down was a major project. Some of the more remote dates could be canceled -- 19 "Rigolettos" at the Met with Sherrill Milnes, for example -- and in others the contract did not specify the repertoire.

The result, Sills says, is that "I have been doing cream puffs for the last two years. I could have gone on for a long time, singing repertoire that I could handle, but I would have been bored. As it is, I have nothing left undone -- nothing to regret. There is no role I wanted to try that I have not tried, no house where I wanted to sing and have not sung, nobody I wanted to sing with and have not sung with." For almost any other artist, this might be an overstatement. For Sills, it is plain, documented fact.

The role she is inheriting took an unexpected escalation this spring when Rudel decided that the original timetable was too slow and he wanted to leave City Opera entirely, making Sills sole director. A position that she hoped to ease into is suddenly becoming her total responsibility. She sounds a little hurt at the suddenness of developments, but understanding.

"Julius is 60 years old," she says, "and he felt that his career was going nowhere. He has been identified with the City Opera for a long time. I think he wanted his own orchestra, and he has it in Buffalo. He wants international recognition, and he has it with opera engagements all over the world. He's entitled to it, and from his point of view he's right. But I hope we will be seeing him back at City Opera, too."

Meanwile, she is wasting no time in making the City Opera her company. "It won't be completely mine until the spring of 1981, when all the new productions are my babies," she says, "but two of the new productions for this fall are mine -- Nicolai's 'Merry Wives' and a set of three one-act American operas that we are calling 'American Trilogy.' If there are any criticisms, I won't duck them even now, but by next spring I will be totally responsible for anything that goes on."

That responsibility includes a season of 160 performances in New York each year and other dates outside New York, with 17 operas in the active repertoire at any given time. As compared to the Metropolitan, one of its partners in the Lincoln Center complex, the City Opera has the reputation of being daring and imaginative on a relatively low budget. It offers a high proportion of operas that are not part of the basic repertoire, and has managed, in the process, to broaden that repertoire considerably. It features American singers rather than the international superstars who are staples at the Met, and it does not have "stars" in the same sense -- people with big names who may fly in for a half-dozen performances before running off to Vienna, Milan or Buenos Aires. Its stars tend to be members of a company in a sense that Met stars are not; the leading singer in one production may be third or fourth from the top in another, and all are supposed to be working together for a common goal rather than cultivating private moments of glory at the expense of the total production.

Under the leadership of James Levine, the Met has been changing some of its policies, particularly in trying more adventurous repertoire and attempting more emphasis on damatic values and an integrated, ensemble effect in its production. But the roles of a major international showcase and a basically American company are different. Sills says she feels her relationship with the Met is more partnership than rivalry.

"Jimmy and I have been friends for 20 years," she says. "Now that he is managing the Met and I am managing the City Opera, we get together regulary regularly and plan our schedules -- to avoid conflicts of repertoire and to provide, between us, the broadest spectrum possible for people who go to the opera in New York. We meet every few months. He will come over to my office and we will eat cookies, or I will go over to his and we will eat fruit and nuts, or we will go out to a restaurant and have Chinese food."

What they discuss, mostly, is the avoidance of conflicts. Levine, for example, is planning a complete Mozart cycle, and the City Opera will postpone its production of "The Abduction From the Seraglio" for several years until the Metropolitan's production has had full exposure and is ready for retirement. "We talk about scheduling conflicts in the less common repertoire," Sills says. "We both keep the bread-and-butter items in our active repertoire, but we are careful to avoid both presenting 'LaBoheme,' for example, on the same evening."

When word of these meetings gets around. New York critics may start talking about the city's "opera cartel," but it seems like a sensible procedure. Given a choice between managing the Met or the City Opera, Sills says she would choose the one she has: "It's a lot more fun. It's a company I grew up with, and I feel very prossessive about it."

Very enthusiastic, too. "In the lobby, after a performance one evening," she recalls, "a woman told me:'It is not fitting that you should applaud so loudly for your own artists.' I don't know what she thought I was -- some kind of claque, I guess. I told her: 'It's fitting, I'm one of the few people in the audience who know what they're going through.'"