Complex sentence: Mariotti made his U.S. debut on Sept.12,2009, conducting "Il barbiere di Siviglia" at Washington National Opera; two months later he performed the same opera, in a different production and with a different cast, at Los Angeles Opera.
Split into simpler sentences: Mariotti made his U.S. debut on Sept.12,2009, conducting "Il barbiere di Siviglia" at Washington National Opera. Two months later he performed the same opera in a different production and with a different cast at Los Angeles Opera.