BOSTON - Two years ago, when Sandra Day O'Connor' was nominated as the first sister to join The Brethren, Reagan called her a "person for all seasons.'' The political commentators, on the other hand, called her "a person for all reasons." She was a two-fer: a conservative and a woman. Now Justice Sandra Day O'Connor has completed her second term at the court with a remarkable finish: She walked down the middle of the road with one foot on each sidewalk. In the court's closing week, O'Connor cast the swing votes in the Norris pension case. First she agreed with one quartet of justices that pension plans can't pay smaller monthly benefits to women than to men. Then she agreed with the other quartet of justices that this decision should not be retroactive, that equality would start from today. As Judith Lichtman of the Women's Legal Defense Fund reads it, "She gave us half a loaf." And this is, in many ways, a decent summary of the First Woman's first two years on the bench. O'Connor has sliced the legal bread on her table in an Intriguing way In most cases, O'Connor voted with conservative Justice William Rehnquist. Indeed their nickname , "the Arizona Twins" could be changed to the Arizona Siamese Twins." She voted with conservatives on the death-penalty issues and on many civil rights issues. She helped narrow the standard for class-action suits and agreed that a plaintiff had to prove an employer's "intent" to discriminate. Finally, in the long-awaited abortion case, I she wrote