The Time Mirror Co., the nation's largest publicly-owned newspaper company and publisher of the Los Angeles Times, said yesterday it has agreed to acquire two more papers.

Two Connecticut dailies. The Advocate of Stamford and the Greenwich Times will be purchased for an undisclosed amount of Times Mirror stock from Gillespie Bros, Inc., the corporation said.

The agreement in principle for the sales was announced jointly in Los Angeles by Franklin D. Murphy, chairman of Times Mirror, and in Stamford by Kingsley Gillespie, president of Gillespie Bros. Inc.

Indstry sources valued the two papers at about $20 million.

THe Advocate, with a circulation of 31,000 publishes Monday through Friday afternoons and Saturday morning The Greenwich Times, with a circulation 14,500, is a five-day afternoon paper. Located in affluent suburbs about 100 miles north of New York City, the two papers are in adjacent communities, but operate independently.

Times Mirror had been negotiating for the paper for more than two years, a company official indicated. Industry sources said that because of income tax considerations, the Gillespie family had sought to sell the papers for stock in a large, ppublicly held company, ruling out some potential buyers.