N.Y. Ad Agency to Open Office in Costa Mesa : Advertising: Two major firms will branch out in the Southland. Aside from serving their just-signed clients, they are expected to compete for other Southern California accounts.

Madison Avenue is sticking its hands into the Southland’s advertising cookie jar again.

Two major New York agencies revealed plans this week to open offices in the Southland. Lowe Marschalk, which recently won the estimated $60-million Prudential Insurance advertising account, plans to open a Costa Mesa office this month. And Young & Rubicam, which just won the estimated $30-million to $35-million 20th Century Fox Film Corp. media-buying account, said it plans to open a Los Angeles office near Fox within weeks.

The chief executives of both agencies insist that their new Southern California offices are only being opened to serve the clients each of them recently won. But the history of many New York agencies that enter the Southland market is that about a year after they establish themselves they go on the prowl for new business. That new business is usually snatched from other agencies in town.

“It’s bad news for agencies like us,” said Greg Helm, president of the 1 1/2-year-old Los Angeles agency Stein Rorbaire Helm. “It brings two more agencies into the fray.”

The move west flies in the face of relatively recent problems the Eastern shops have experienced in the Los Angeles area. In 1987, William Esty left town when it lost the Nissan account to Chiat/Day/Mojo. And in 1988, Scali, McCabe, Sloves closed its Los Angeles office less then two years after it opened.

But the two newcomers say they certainly expect to make it in the Southland. And they will do that, they say, by concentrating on the clients they have just won.

Lowe Marschalk expects to hire up to 15 employees for its first office in Southern California, said Andrew J. Langer, chief executive. The office will primarily serve the Costa Mesa-based Prudential Real Estate Affiliates.

But Langer has not ruled out the possibility of eventually taking on other business. “If someone in Los Angeles came up to us and said, ‘Would you like to handle our $50-million business?’ we’d be hard pressed to say no.”

Meanwhile Peter Georgescu, president of Young & Rubicam Inc, said the agency will focus on Fox. “We’re ready to go to work.” Georgescu did not know how many people the office would employ, but he said some Young & Rubicam employees in other offices would likely be transferred to it.

Certainly, Young & Rubicam knows firsthand how difficult Los Angeles can be. It has opened an office--and closed it--in Los Angeles once before.