(Reuters) - Two major shareholder advisory firms urged investors to vote against billionaire investor David Tepper's hedge fund demand that Allergan Plc immediately split the roles of chief executive officer and chairman, in a boost to the Botox maker. The recommendation from Institutional Shareholder Services LP and Glass Lewis & Co against the proposal of Tepper's Appaloosa gives firepower to Allergan Chairman and CEO Brent Saunders ahead of the drugmaker's shareholder meeting on May 1.