Two of the biggest savings and loans in New York announced a $1.6 billion merger Friday, consolidating their loan portfolios in an environment of increasingly thin lending margins. New York Community Bancorp ( NYB), a $12 billion-in-assets thrift based in Westbury, N.Y., will exchange stock worth $20.33 for each share of Roslyn Bancorp ( RSLN),a $10 billion-in-assets thrift based in Jericho, N.Y. The combined company will be slightly larger than New York's currently biggest thrift, Astoria Financial ( AF). The companies expect the transaction to be immediately accretive to earnings. They also announced a concurrent balance-sheet restructuring in which they will sell off some $3.5 billion of securities, probably mortgage-backed bonds that are traditionally owned by home lenders. North Fork Bank ( NFB), another New York-area savings and loan, announced a similar balance-sheet cleanup Tuesday. North Fork's announcement was coupled with an earnings warning that cited a contracting lending margin, reflecting the 13 consecutive interest rate cuts carried out by the Federal Reserve.