NEW YORK -- Howard Koeppel greets a visitor at his posh Upper East Side apartment, with its million-dollar views of midtown Manhattan.

``Welcome to Gracie Mansion annex,'' he says with a grin. ``I'm the first lady.''

He's only half-kidding. In a turn of events tailor-made for this city's tabloid headlines, Republican Mayor Rudolph Giuliani separated from his wife, Donna Hanover, and then did something that has even many I've-seen-it-all New Yorkers baffled:

He vacated the mayoral mansion to become roommates with a gay couple and their pet Shih Tzu, Bonnie.

Since June, Giuliani and his bodyguard have taken residence in the elegant 3,000-square-foot home of old friend Koeppel, a 64-year-old Queens car dealer, and his 41-year-old partner of 10 years, Mark Hsiao.

Giuliani brought only a few suits and some toilet articles. He sleeps on a bed laden with throw pillows, one proclaiming: ``It ain't easy being king.''

``He hasn't exactly moved in,'' Hsiao says. ``We prefer to call it `crashing.' He makes his own bed, takes away his dirty laundry and tiptoes in when he comes in late at night. Sometimes I feel like I have two husbands.''

The unlikely living situation has opened a window into the world of a mayor often seen as distant and contentious.

Giuliani left Gracie Mansion after a judge ruled in May that he could no longer bring his girlfriend, Judith Nathan, to the mayor's official residence as long as his estranged wife and their children continued to live there.

While Giuliani has declined to speak publicly about his new living arrangement, Koeppel and Hsiao candidly offer intimate, day-to-day details about living with a politician whom appointment-seekers sometimes must wait weeks to see.

Many nights, the roommates watch television together in the three-bedroom, five-bath apartment furnished with Chagall paintings and a life-size David statue.

Giuliani prefers such HBO shows as The Sopranos and the New York-based comedy Sex in the City to what he calls the more morbid fare of Six Feet Under.

When Giuliani dives into his morning cereal, the three discuss everything from opera to selling cars in Queens, Koeppel says. ``He thinks my business is a good barometer for what's going on in the city.''

Koeppel sometimes jokingly asks the mayor what time he'll be home. And Giuliani has been known, after bidding goodbye to a growling Bonnie, to kiss both new roommates on the cheek.

``It's an Italian thing,'' offers the no-nonsense Koeppel, who counts among his close friends comedian Jackie Mason.

Adds Hsiao, a Julliard-trained pianist who works at the city's Department of Cultural Affairs: ``It's also a gay thing.''

Longtime Democrat Koeppel met then-U.S. Attorney Giuliani in 1989 before his first bid for the mayor's job. He was so impressed with the man that he switched parties and helped elect him New York mayor in 1993.

Friends with Giuliani through the 1990s, Koeppel and Hsiao saw signs of trouble in the mayor's marriage. ``But he never said anything, and we'd never ask. . . . We didn't really know things were so bad until it hit the papers.''

After discussing the matter, the pair invited Giuliani to move in.

And after considering the offer for two months, he accepted.

Gay activists say the arrangement has shown a positive side to Giuliani and to gay relationships.

``This is the mayor who introduced the law making domestic partnerships legal in New York,'' says Christopher Lynn, 51, a gay lawyer who works in the Giuliani administration.

``His attitude toward gays is only unusual for people who don't know Mayor Giuliani's record.''

Koeppel says the mayor accepts his new roomies for who they are.

``Even though he's a devout Catholic, he doesn't see us as a gay Jew and a gay Chinese guy,'' Koeppel says. ``He's never said to me: `Couldn't you find a nice Jewish man?' Something my own relatives might have said.''

Koeppel jokes with the mayor that if things don't work out with girlfriend Nathan, he can ``come over to our side.''

This, Koeppel says, is where Giuliani blushes and promises to think about it.