The opening night at Tanglewood (the Boston Symphony's summer home) in July was to be the first time Maestro James Levine would conduct the Boston Symphony in Mahler's soaring second symphony. But back surgery forced him to put down the baton, clearing way for Michael Tilson Thomas to fill in, to great acclaim. In October, BSO listeners finally get to hear Levine's take on Mahler's 2nd; a rare opportunity to compare two conductors' interpretations of a work with the same orchestra only a few months apart. 10/7-10/12