Complex sentence: He decorated the front of the building with four columns each surmounted by a caryatid and commissioned Injalbert to sculpt them and Paul Landowski was commissioned to carve a bas-relief.
Split into simpler sentences: He decorated the front of the building with four columns each surmounted by a caryatid. These were the work of Injalbert and Paul Landowski was commissioned to carve a bas-relief.