a picture is worth a thousand words:
http://www.atasoyotomotiv.net/image/...OR-800x680.jpg
the shot above illustrates that the pump has an electrical connection for regulation
on the S55, the pumps are regulated separately, and one is secondary, engaged sequentially by the DME
more pics:
https://www.google.com/imghp?hl=en&q=bosch+hdp+5