Not sure of the mechanics, but I read the series thing in a Bassplayer review---said the decrease in volume when switching to a single pup was because when both were on, it was in series.
I'm more of a musician-type so I can really only tell about the tone...both is good'n hot / bass only is o-k / treble only is useless far as I can tell...it's real close to the bridge so it sounds kind of anemic like a capped Rick bridge pup.
Seems like I remember reading something that said they wired the pickups like that because for some reason it allowed them to use that cheap little toggle switch as the only switch for all of their guitars and basses?