Complex sentence: During this time, the theatre was under the ownership of the "Stoll Moss Theatres Group" and under the management of both Margaret and David Locke, who were both major shareholders of Stoll Moss.
Split into simpler sentences: During this time, the theatre was under the ownership of the "Stoll Moss Theatres Group". Which at the time the theatre was under the management of both Margaret and David Locke, who were both major shareholders of Stoll Moss.