Brockton (Mass.) Retirement System is searching for two managers to run about $30 million to $50 million each in enhanced S&P 500 index portfolios, confirmed Harold P. Hanna Jr., executive director. Officials at the $378 million fund want to shift about $200 million of the plans $205 million active domestic equity allocation to enhanced indexing to boost returns. Consultant Evaluation Associates is assisting. Mr. Hanna said selections could be made as early as June.