Complex sentence: American composer Michael Kamen, who had contributed to "The Wall", oversaw the orchestral arrangements; stood in for absent keyboardist Richard Wright; co-produced; and mediated between Waters and Gilmour.
Split into simpler sentences: American composer Michael Kamen, who had contributed to "The Wall", oversaw the orchestral arrangements. He also stood in for absent keyboardist Richard Wright, co-produced, and mediated between Waters and Gilmour.