A new Tennessee law that allows small businesses to form cooperatives to buy health insurance went into effect, but only one insurer has developed a product under the law, whose supporters had hoped it would stabilize rates by spreading risk. BlueCross BlueShield of Tennessee is seeking groups to pilot up to 10 co-ops statewide, starting in March. But a spokesman said it isn't clear how interested small businesses will be. Insurers and businesses in other states that passed similar laws recently have been slow to sign on, and some of them question the potential for savings.