High-style units. Unheard-of price.

These Commonwealth Avenue sisters stand out like Parisian ladies at a tea party of Boston Brahmins.

The adjacent Back Bay town houses are rare examples of ornate Beaux-Arts architecture in Boston and, by coincidence, are both for sale. One of them, priced at nearly $15 million, could easily set a record as the city's most expensive single-family home.

"They're like that unique waterfront property that comes on the market on Cape Cod that's been in someone's family forever, and it's exquisite," said Back Bay real estate agent Ken Tutunjian.

The twin sales offer a peek into the rarified world of the ultra-high-end housing market. Neither is having an open house; potential buyers for one house must present certification of their wealth from a financial planner just to tour the home, and then, to weed out all but serious bidders, must produce a $500,000 deposit to be able to bid.

Located at 128 and 130 Commonwealth Ave. , on the south side of the street, between Clarendon and Dartmouth, they are examples of an architectural style that flourished from 1885 to 1920 in Paris and Manhattan -- but is uncommon in tradition-bound Back Bay where understated red brick and brownstone town houses are the typical style.

A showy style that was rich in detail but sometimes given to frothy ornamentation, Beaux-Arts was favored by the rich in America at a time when wealth was displayed ostentatiously. The Vanderbilt Breakers mansion in Newport is perhaps the most famous Beaux-Arts residence in the United States.

Both Commonwealth Avenue town houses are made of white limestone, with garlands sculpted into their facades, Juliet balconies, and top-floor dormers trimmed in ribbons of copper turned green. Architectural historian Bainbridge Bunting once wrote the Beaux-Arts homes seemed to Bostonians to be unnatural imports from New York City.

"They seem rather out of place," he said in his 1968 book, "Houses of Boston's Back Bay," and "their presence serves to remind us of Boston's resistance to Knickerbocker fashions and of her overwhelming allegiance to her native traditions."

Their interiors are drenched in original mahogany paneling and feature enormous, arched windows overlooking the boulevard, floral wall and ceiling carvings, and 13 fireplaces between them. Outdoor parking spaces at the rear of both buildings have heated pavement.

Auto dealer Herb Chambers is selling 128 Commonwealth. Ave. for $14.95 million. His neighbor, businessman Deepak Kulkarni, elected to have Chicago auctioneer Sheldon Good & Co. troll among an international set of potential buyers to get the highest possible price. The auction is set for June 21.

The homes may appeal only to an exclusive set of ultrawealthy buyers who collect houses the way others collect rare wines or vintage baseball cards. They are far too expensive for developers to buy and chop into condos for quick profits, said agents and developers.

The Commonwealth Avenue mansions are among five houses currently listed for sale at $10 million or more in Back Bay and Beacon Hill, according to the Listing Information Network, or Link. The current sales record is $9.12 million for 51 Commonwealth. Ave. in 2003. "Three years ago, $10 million was the ceiling, and now we're seeing a higher ceiling," said Link's president, Debra Taylor Blair.

Finding one eligible buyer -- much less two -- will be a challenge. The simultaneous listings may affect the price each gets, though agents said it is difficult to predict when talking about such expensive properties and prospective buyers with millions to spend on real estate.

"They're competing with each other," said Boston real estate agent John Neale. "It's certainly not going to make them more valuable."

The Beaux-Arts homes may also not be to everyone's taste. Competitors include the contemporary luxury condominium towers sprouting all over downtown Boston, such as One Charles , the Residences at the InterContinental, and the Residences at the Mandarin Oriental Hotel. These properties are rapidly filling up with corporate executives, entrepreneurs, and wealthy suburbanites, drawn by the sturdy construction, modern materials, and amenities such as dog walking or fresh steaks delivered to roof-top decks for grilling.

"People want places that are less noisy with modern interiors," said David Epstein, president of Abbey Group in Boston, which is developing 45 Province, a glass and terra cotta condo high-rise near Boston Common and King's Chapel.

Indeed, Chambers is planning to move to the Mandarin, where he committed to a buy a $7.5 million condo at the same time he bought the Commonwealth Avenue town house on a whim in June 2004 for $8.65 million. "It's a great entertaining house, but I don't do a lot of entertaining," said Chambers.

There are about a dozen Beaux-Arts residences in Back Bay, including another pair at 426 and 428 Beacon St., said William Young, senior preservation planner for the city's Back Bay Architectural Commission. The two Commonwealth Avenue homes did not start out in the Beaux-Arts style; rather they were built as brownstones in 1882. In 1909, the owners redid the facades of the buildings in Beaux-Arts style, according to Young.

"They're like Austrian pastries," he said.

The current owners also revamped the interiors to suit their different tastes.

When Chambers purchased 128 Commonwealth Ave. in 2004, it had previously been converted to three condominiums. In 2005, he began restoring it to a single residence, at a cost that was estimated at $750,000 in city records but, he said, was "much more than that."

He installed a two-car underground garage, with an elevator to the fourth floor, where a terrace offers close-up views of the towering John Hancock and Prudential buildings.

Plaster ceiling decorations are trimmed with gilt paint. Elaborately carved new features include a mahogany mantel and ornate balustrades, and the master bathroom has floors and walls made of onyx, a more luxurious stone than the marble found in garden-variety upscale homes. The rarely used kitchen stove is by La Cornue, handmade in France.

Kulkarni's renovation hewed meticulously to the period. The uninhabited house was in a "dilapidated state" and lacked running water when he bought it in 1992, a low period in local real estate markets.

He hired New York architect Alan Wanzenberg to undertake a restoration, which added modern features such as an elevator.

The enormous foyer gives way to a five-foot-wide staircase with wide, open landings on each floor that give the residence an airy feel. Wanzenberg hired an artisan to hand polish the original walnut paneling, which was "book matched" in the 1800s so that the wood grains align -- a rare feature today. He also had door hinges machined in Germany that work so smoothly he can move heavy doors with one finger, he said.

The real estate community is watching the sales closely. Chambers put his on the market the old-fashioned way, retaining Back Bay agent Beth Dickerson in March. Kulkarni, meanwhile, has created a buzz in Boston's real estate community by hiring outside auctioneer Sheldon Good, which sold the Miramar waterfront estate in Newport last year for $17.15 million.

"High-quality art gets sold in auction. High-quality companies get sold in auction," Kulkarni said. "A lot of things that are valued are sold in auction, and this building is a piece of art."

Kimberly Blanton can be reached at blanton@globe.com.