The Amalgamated Clothing and Textile Workers Union scored another quick victory yesterday in its campaign to isolate the J.P. Stevens Co., which is fighting unionization.

Ralph Manning Brown Jr., chairman of the New York Life Insurance Co., and James D. Finley, chairman of J.P. Stevens and Co., announced they are resigning from the boards of each other's companies.

The move came only one day after the union announced two candidates to challenge Brown and Finley for re-election to the New York Life board next April unless the corporate interlocks between the two companies were severed. It also follows several related resignations last spring.

"The union's involvement of New York Life in the Stevens controversy places me in a position of conflict between the interest of New York Life and Stevens," Brown said in a statement, adding that, "under the circumstances, I must consider the interests of New York Life to be Paramount."

He noted that the union's campaign to get New York Life's list of its 6 million policyholders and to contest the directorships would have proved "very expensive and burdensome" to the company, and said he therefore was resigning from the Stevens board "with deep regret."

Stevens Chairman Finley said that, "Under the circumstances, I believe that I have no alternative other than to resign" from New York Life's board.

He added that he was tendering his resignation "in both sorrow and anger because I firmly believe that the tactics engaged in by the union to force my resignation and the resignation of Manning Brown are improper and unconscionable."

Finley pointed out that Avon Products Chairman David Mitchell had been forced by the union to resign from the Stevens board last spring.

"The loss as directors of David Mitchell and Manning Brown is a scrious and tragic matter for Stevens," he said. "It would be unrealistic to regard it as anything less than that."

The union, meanwhile, called the resignations "an important step forward in our campaign to expose and sever the interlocking directorates that have nourished the J.P. Stevens Co. in its vicious campaign to deny its workers their rights to social and economic justice."

The textile company contends that the union over the last 15 years has failed to get the support of enough Stevens workers to qualify for an election on whether to unionize. The union, in turn, claims the company has intimidated and harassed workers to frustrate organizing efforts. And Stevens, the country's second-largest textile manufacturer, repeatedly has been cited by the National Labor Relations Board for labor law violations.

Ray Rogers, head of hte union's so-called "corporate campaign" against Stevens, said the resignations were "an indication of the increasing alienation of the corporate and Wall Street community from J.P. Stevens."

The union also is campaigning for the resignation of another Stevens director, E. Virgil Conway, who is chairman of New York's Seaman's Bank for Savings.

Rogers noted that S.J. Weinberg Jr., the managing partner of Goldman Sachs and Co., one of Wall Street's largest investment banking firms, also sits on the Stevens board, but he has not yet been targeted by the union.

"We're meanwhile also concerned about institutions that hold and control large amounts of J.&P. Stevens stock," he added, pointing to Bankers Life of Iowa and Morgan Guaranty Trust of New York in particular, but declined to describe what tactics, if any, he had planned against the two big financial institutions.

Besides the resignations of Avon's Mitchell last spring from the Stevens board, Mitchell and Stevens Chairman Finley both withdrew from renomination to the board of Manufacturers Hanover Trust after the big New York Bank was threatened with the loss of union business.