Hundreds of singers from Boston-based choirs and choral societies and the two hundred members of the New World Chorale, under the direction of Choir Director Holly Krafka and Conductor Christopher Wilkins, will combine to form the “One City Choir” for one night only in celebration of the opening night of the Boston Landmarks Orchestra’s 2012 Landmarks Festival at the Shell.The orchestra and choir will perform works by Copland, including selections from Old American Songs and The Tender Land.