Found this page interesting. It has detailed photos of a secondary compressor installation. Nice to see what it would like.
http://proairllc.com/image/data/docs/Promaster/09%20000%20420%20Install%20Instr%20DPM%20Comp.pdf
http://proairllc.com/image/data/docs/Promaster/09%20000%20420%20Install%20Instr%20DPM%20Comp.pdf