Sixteen months after Gov. Deval Patrick signed a law clearing the way for small businesses to band together to buy health insurance, the Division of Insurance said yesterday it has certified two organizations as the first group purchasing cooperatives. The certification, effective Jan. 1, allows the Retailers Association of Massachusetts and the Massachusetts Association of Chamber of Commerce Executives to begin talking with the state’s commercial health plans about offering new lower-cost insurance products to the tens of thousands of members they hope to attract to their cooperatives.