(Reuters) - Two leading shareholder advisory firms threw their support behind Allergan Plc's management by recommending that investors vote against billionaire investor David Tepper's proposal that the Botox maker immediately split the roles of chief executive officer and chairman. The recommendations of Institutional Shareholder Services LP and Glass Lewis & Co against the proposal by Tepper's hedge fund Appaloosa LP give critical support to Allergan Chairman and CEO Brent Saunders before the drugmaker's shareholder meeting on May 1.