The Paris water system contains pairs of pipes -- potable for drinking, and non-potable for cleaning. (I'm not sure if the non-potable is just for street cleaning, or if it's used for toilets too.)
It seems so much more obviously cost-effective, I wonder why that never took off anywhere in the US, as far as I know?
I know of at least one sub-division where non-potable water is connected to yard irrigation systems, so the concept is not totally unheard of. Nothing as sensible as the system you outline in Paris, though.