I hadn't thought of it that way, but any pump can only manage to a certain head, and as they are just off an incline they are at different heights. I assumed the lower pumping from a 'sump' below up to a cistern where the upper pump possibly operated on a float switch. I assumed the dam was simply down to that passage being out of use but cutting a feeder.
In conventional mines, if that is a proper description, upper shaft cisterns collect from the adjacent level rather than let the water all go down to the bottom to then have to be raised.
This accounts of course for the increase in diameter of the rising mains as one progresses up the shaft.
Possible explanation of the use of the word "combined"?
I would like to hear from divers about anything left in situ, photos even better.
My avatar is a poor likeness.