Viacom, planning to split into two companies next year, named Shari Redstone as non-executive vice chairwoman of both units and said investors will receive half a share in each new business for every Viacom share.

The boards of the new Viacom and CBS will share three other common directors, including Redstone’s father, Chairman Sumner Redstone, New York-based Viacom said yesterday in a regulatory filing that gave details of the breakup.

Sumner Redstone is splitting Viacom, the third-largest American broadcast company, to revive its shares, down 10% this year after tumbling 18% in 2004. The new CBS will contain broadcast TV and Infinity radio and carry about $7 billion in debt and most of Viacom’s pension obligations. The new Viacom, with cable networks such as MTV, will have more than $3 billion in debt. Viacom shares slipped 2.4% yesterday to a four-year low.

“Because you can’t buy the new media of cable-TV networks without buying the old media, investors are staying on the sidelines,” an analyst at Soleil Securities in Pasadena, Calif., Laura Martin, said. She rates the shares “buy” and doesn’t own any. “The catalyst is going to be when you can buy the separately traded pieces.”

The new CBS posted sales growth of 7.3% last year, compared with an 11% sales increase for the new Viacom, which will also include Paramount Pictures and cable-TV networks such as Comedy Central and Nickelodeon.

Viacom “thinks they can accomplish more as separate companies,” a senior director of corporate finance at Fitch Ratings, Brendan Buckley, said. “If Viacom is successful, it’ll be interesting if other large media conglomerates start to contemplate a similar strategy.”

Since Mr. Redstone first proposed the breakup in March, New York-based Time Warner, the largest publishing and broadcast company, has come under pressure from shareholders led by Carl Icahn to shed some assets, and John Malone’s Liberty Media has spun off its cable-TV company Discovery Communications.

Viacom’s Class B shares fell 77 cents to $31.83 at 4:18 p.m. in New York Stock Exchange composite trading. It’s the lowest close for the shares since September 2001.Time Warner fell 10 cents to $18 and Walt Disney, the no. 2 American entertainment company, dropped 33 cents to $24.23.

The new Viacom will have the ticker symbols “VIA” and “VIA/B” and CBS will have “CBS” and “CBS/A.”

“We recognize the need to adapt to a changing competitive environment,” Mr. Redstone, 82, said in a letter to shareholders that was included in the filing yesterday. The separation will create “two nimble and focused companies,” he said.

Viacom didn’t give a date for the split.

Shari Redstone, 51, is being groomed by her father to take greater control of the company. She was named vice chairwoman of Viacom in June after serving on its board for 11 years. In 1993 she ended a career as a corporate lawyer to run National Amusements, a closely held theater chain started by Sumner Redstone’s father in 1936.

The decision to separate the company also resolved a succession battle between co-presidents Leslie Moonves and Tom Freston, who were vying to replace Sumner Redstone. Each will become chief executive officer of one of the businesses.

Mr. Moonves will take CBS, which will include Viacom’s billboard advertising unit, the UPN broadcast TV network, and publisher Simon & Schuster. CBS will pay a dividend, while the new Viacom will spend money making acquisitions, Viacom said.

Sales in the first half of 2005 were $7.11 billion, and profit from continuing operations was $653.3 million, or 81 cents a share.

The CBS business’s sales rose to $14.5 billion in 2004 from $13.6 billion in 2003.The unit posted a loss from continuing operations of $16.3 billion, or $19.03 a share, after writing down the value of its Infinity radio and billboard assets. The business posted a profit in 2003 of $1.12 billion,or 63 cents a share.