Complex sentence: The company constructed another theatre, Sidney Harman Hall, which opened in 2007 in the lower part of an office building in the quarter, and the two theatres were joined to become the Harman Center for the Arts.
Split into simpler sentences: The company constructed a second theatre, Sidney Harman Hall, which opened in 2007 in the lower part of an office building in the quarter. At the same time, the two theatres were joined to become the Harman Center for the Arts.