hmmm... good thing you are telling me why these things are a bad idea... let's try this one...
ok, so we have a cistern, a tank in the attic, and a house. Nothing is to scale, PLEASE do calculations if you are going to do something like this, there are probably easier ways to accomplish the same thing, I am not qualified to design these sorts of systems, etc. etc. etc.
Latest idea:
IF cistern is over 50% full, pump 1 operates every time valve 1 is tripped, topping off the tank
IF the cistern is less than 50% full, pump 1 no longer operates, pump 2 AND city water operate every time valve 2 - city valve are tripped, mixing 50/50 and filling the tank (More likely there would be one valve tripped that started both)
IF the cistern is dry (or below a certain level), pump 1 and pump 2 no longer operate, city water operates every time city valve (or common valve) is tripped, filling the tank
IF you get tired of my ideas, just let me know.
Have a good Earth Day!
Wyatt