At the Fillmore School in Georgetown, when a student gets sent to the principal's office, he can't be sure which "half" of the principal it will be because two women are sharing the job.

In California, a young man was offered the job of comptroller of a large school but wanted more time with his family than the high-pressure position would allow. Since his wife had skills similar to his own, they offered themselves as a team, and got the job.

At the admissions officer of Goucher College in Towson, Md., a job cut reduced three positions to one. The three women who held the jobs decided to offer themselves for the one job, and they got it.

Such examples of job sharing are increasing, although they are still rare. Getting an employer to agree to such an arrangement and then making it work well takes a special combination of skills, personalities, work environment and luck, according to those who are in the thick of the movement.

Many employers and supervisors have resisted job-sharing and other kinds of reduced-hours arrangements inmost higher-paying occupations because of concern about extra costs and administrative inconveniences, and because of the ingrained belief that employees who want to work part-time are not as committed or skilled as full-time workers.

For the three working mothers at Goucher College, the arrangement works "because we're compatible, we think alike," says Janet Kantor, one of the three. "Each of us knows what the others are doing and is familiar withthe people the others deal with."

Their job involves setting up a program in which college alunae speak to prospective students and generally serves as liaison between the alumnae and the admissions staff.

"One of us is almost always available. Sometimes, we work at home at night, or very early in the morning, but if one of us has a sick child or something like that, the other two will cover for her."

Sharing a job can be "tricky," according to Maureen McCarthy, of the Committee on Alternative Work Patterns in Washington. "You either have to define pretty well what each person's responsibilities are, or they have to be very compatible, or you can get into trouble."

"You have to guard against the staff playing one of you against the other and things like that," said another Washington professional.

One of the few settings in which the mechanics and the effects of such work arrangements can be observed in a variety of professional and para-professional jobs is Wisconsin. An experimental, federally funded program called Project Join (Job Opportunities and Innovations) has taken 30 full-time state government positions and transformed them into 60 part-time or shared positions. There is one year left in a two-year grant. Its initial intention was to include women and other categories of workers who couldn't work full-time.

The first surprise according to program chief Carol Lobes, was in the breadth of employee interest. "We had in essence excluded men, unless they were older workers with special needs. But we found some young, career men interested, too, because of family interests or for various other reasons."

Supervisors, on the other hand, have resisted the plan. "That has been an uphill struggle," Lobes said. "We found an interim step necessary. Supervisors are more willing to first allow an existing employee to reduce hours and then hire a second, rather than taking one vacant position and hiring two new people."

Not every job is adaptable to part time or job sharing, but, according to Lobes and others, in jobs where an alternative schedule could be worked out, supervisors' reactions have been generally favorable.

"It doesn't make management's job any easier, but I think it's a good thing overall," says supervisor Michael Lovejoy, of the staae's Office of Policy and Program Analysis. He oversees a staff of 46 business administration professionals, which now includes one who works half-time.

There is some "subconscious screaming," he adds. "One management technique is to load people up with too much work, and then let them - if they've good - sort cut what's important. But you look, 'I can't load them up with that.' I think you tend to give the part-time persons the more structured, projects. The full-timers tends to get the flaky, and often enjoyable and creative projects."

Carol Gannon, a medical technician for the state of Wisconsin, said the biggest problem in her job-sharing situation is "communication. My 'other half' and I leave a lot of little notes for each otherabout what we've done."

"Job sharing allows the employer to get two different people who, taken together, make one person who's perfect for the job" says Jor Jan Borlin, a community services technician who gathers data on the state's community action programs. "I'm better at some things and the other person has strengths in other areas."

She wanted to work part-time because she needs time for weekly therapy for a paralysis in her legs caused by an accident six years ago.

The most strenuous objections by employers concern the added costs of fringe benefits for part-time workers, which can be several hundred dollars a year or more extra per worker.

Public agencies do not pay the same penalty as private employers in unemployment compensation or Social Security. But many public agencies are required to pay all employees full health benefits.

Many collective bargaining agreements require a full day's pay for a worker called in for any part of the day. Or they require full fringe benefits to be paid to all workers. These, too, raise the cost of part-time employment.

In a preliminary cost study of the Wisconsin program, according to co-director Mary Cirilli, "the state seems to be saving money based on the salaries and benefits overall." She said the extra costs in benefits are offset by lower salaries being paid to some half-job holders.

Carol Gannon, for instance, had been the full-time head of a state medical project. When she asked to share her job, the other half was assigned only research work at lower pay.

Some employee benefits, such as paid vacation time, can be prorated in job-sharing arrangements. But other employee benefits that are mandated by law for workers can raise an employers' cost if there is job sharing.

It is in the category of "fixed cost" benefits mandated by law for each worker that costs increase for part time workers, though this varies. For private employers, for instance, unemployment insurance contributions cost more for two part timers than for one full timer. And yet many part timers don't work enough hours to collect a share of unemployment compensation. (If payments are required on the first $6,000 of annual salary, the employer pays one amount for one $12,000 salary and twice that amount Social security costs present a for two $6,000 salaries.) similar disadvantage in certain pay brackets.

"Employers do have a gripe right now" about the cost of part-time arrangements, said Wisconsin state Rep. Midge Miller, who has been active in pushing for more flexible work arrangements.

"Well, men made these rules. They can change them."

Next: Factory tailors hours to inner-city mothers.