Why are there two? Do they both operate in tandem. Is one considered the "high demand" pump and turns on when appropriate?

And how common is it for them to leak? My forward one is not really leaking per say, but the underside is varnished up pretty good. the other looks clean as can be.

For replacement on a 560 SEL, what is recommend? (I am sure a search will say, but as long as I am making a post.)

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car