Two of the leading accounting firms in the Southeast, Greenville, S.C.-based Decosimo and Chattanooga-based Elliott Davis, are merging to create the fifth largest firm in the region with annual revenue of $108 million. The combined firm will be known as Elliott Davis Decosimo when the merger goes into effect Jan. 1, 2015. Its footprint will stretch across Alabama, Georgia, North Carolina, Ohio, South Carolina, Tennessee, Virginia and the Cayman Islands. Rick Davis, managing shareholder of Elliott…