Complex sentence: There they performed their production of "Twelfth Night" in which Marda played Maria and Gwen, Olivia, and the play "Quality Street" by James Barrie.
Split into simpler sentences: There they performed their production of "Twelfth Night" in which Marda played Maria and Gwen played Olivia. They also produced and acted in the play "Quality Street" by James Barrie.