Wouldn't salt water in toilets require two sets of pipes leading into each home? Seems like an expensive proposition unless you're building a new community from scratch.
Similarly multiple out-going pipes to split off various levels of grey waste (some direct to garden, others, to soakways, or to septic processing systems).
Capex for these kinds of systems is ultimately dwarfed by savings on opex - just like we find with mandatory double-glazing & other insulation regulations.
Regulations lag behind optimum configuration, though parts of EU, especially in the north, legislated triple-glazing (f.e.) quite a few years ago (can't find a cost analysis of break-even points, unfortunately). I'm sure there's plenty of assumptions wrapped up in these regulations around estimated abode longevity, energy pricing over that period, etc. But as to multiple water sources into abodes, I haven't seen much formal work on it. I think the idea gets touted in London periodically - it'd be a nice solution to the dual problems of high water table and (surprisingly) occasional potable water shortages. But retrofitting this to an existing, massive, badly planned metropolis would be a nightmare.
now what should be more easily done is require building codes to insure large complexes have separate gray and black water pipes so that easier to treat water is routed better.
Are there examples of this done on a larger scale? I know facilities processing their waste-water onsite sometimes have this, but has it been done on a town/city level?
If your target it a farmer in the middle of nowhere you have twice as many pipes, twice as many pumps and no real ways to leverage economies of scale. Thus farmers typically have their own private well: they can drill a new well every 10 years for less than the cost of putting a pipe from the nearest city to their house. (wells generally last more than 10 years, but you have to replace the pump once in a while)
If you are targeting a downtown then there are high rises all over which increases the population density. The cost of putting in two sets of pipes is not much more than one larger pipe, and the pipes are amortized over everyone in the building on both sides of the roads - figure and extra $.50/month to have the second set of pipes: cheap enough that nobody will care.
The above are two different extremes, the only question is where along the spectrum is your question asked?