For more than three decades, the Turners and Usdans lived side by side in two of the smallest 19th-century brownstones in Manhattan. Now, they are ready to move on, again in tandem: They put their twin, 12½-foot townhouses on East 78th Street on the market together.

In the arithmetic of New York real estate, one plus one often equals far more than two, and the families are working together to sell their tiny homes to a single buyer as a 25-foot-wide mansion. The asking price: a combined $11.95 million.

From the street, the compact townhouses are mirror images of each other. With the exception of the building numbers, one in brass, the other in ceramic tile, they already appear to be a single building.

The rear yard of 155 E. 78th St., one of the twin brownstones for sale. Ramsay de Give for The Wall Street Journal

Inside, the rooms measure a maximum of 11 feet wide. They are smaller where there are fireplaces or built-ins, and they are far narrower in the center, where one can just about touch a finger on a stair banister and the other on the far wall.

Yet the interiors are remarkably different: one is glitzy and modern, the other preserves Victorian touches.

In Manhattan, there are only three homes under 12 feet wide, with the slimmest at nine-and-a-half-feet on Bedford Street in Greenwich Village, according to city tax records.

But there are about 60 houses between 12 and 13 feet wide, the logical result of a grid of streets laid out in 1811 with standard lots that were 25 feet wide and could be divided. These small lots can be a tough sell, in the age of McMansions in the suburbs and sprawling penthouses in the city.

So the hope on East 78th Street is to sell the well-preserved houses on a quiet residential block off Lexington Avenue to a family or a developer looking to create a single 25-wide mansion by breaking through the wall between the houses.

"Somebody could create a gorgeous piece of architecture," said Anne Snee, a Corcoran broker who helped the Turners buy their house in 1977 and is now working with both families.

It is not clear whether an eventual buyer would remove a staircase and open up and preserve the existing features, or tear down both houses and start from scratch. The houses are just outside the bounds of an historic district, and the property can be developed for commercial as well as residential use.

This has raised worries on the Upper East Side.

"We are always concerned when older and more or less intact buildings are getting torn down to be replaced with something new on a block that has a lot of character," said Tara Kelly, the executive director of Friends of the Upper East Side Historic Districts.

Pamela Usdan, a television producer, and her husband, Morton, a yarn manufacturer, bought 153 East 78th Street in the spring of 1978 for $185,000.

That was only about six months after Jon Turner, a professor at New York University's Stern School of Business, and his wife, Margaret Helfand, an architect and former president of the American Institute of Architects' New York Chapter, paid $165,000 for their slightly smaller house.

The two families kept their rear yards open, separated by a row of hedges with an opening, so that they could borrow each other's yard when holding parties or barbeques.

The couples spent many New Year's Eves together and were friends over the years, according to Mr. Turner. They hired a contractor to restore both facades at the same time.

In the late 1970s, as the city recovered from a deep fiscal crisis, real-estate prices were depressed. Mr. Turner said he and his wife bought the townhouse on the rebound after being outbid by a few thousand dollars on a co-op on Central Park West with a 50-foot long park view. Their bid on the co-op: $78,000.

The Usdan house is fiercely modern with black walls for accent and track lighting; walls and even a ceiling in one room are covered with mirrors to expand the sense of space.

Next door, Ms. Helfand, though known for her contemporary designs, restored and preserved ornamental plaster details in the ceilings, and saved old molding and etched glass pocket doors in the living room.

After his wife died of cancer in 2007 at the age of 59, Mr. Turner decided to spend most of his time at an apartment he owns San Francisco. Mr. Usdan had died the year before at age of 70.

Last June, Mr. Turner put his house on the market for $5.75 million, and cut it to just under $5 million this fall.

A few weeks ago, his neighbor decided it was time for her to sell as well, and the combined listing went on the market last week.

Despite the slender width, Mr. Turner said the rooms at either end of the staircase don't feel small, and room for room, the spaces are as large as most of those in Manhattan apartments.

The difference, he said, is that unlike in an apartment, the rooms are stacked one on top of another.

And he said the stair climbing was better than going to the gym.

"I miss the stairs," he said. I went up and down those stairs 15 or 20 times a day. If you do that you're going to have great legs."

Write to Josh Barbanel at