Complex sentence: Copley produced a second, full-size replica for himself, which is in the Museum of Fine Arts, Boston, and a third smaller version, with a more upright composition, which is in the Detroit Institute of Arts.
Split into simpler sentences: Copley produced a second, full-size replica for himself, which is held by the Museum of Fine Arts, Boston. His third and smaller version, with a more upright composition, is held by the Detroit Institute of Arts.