Parallel sewage/runoff systems seem to be good ideas to me. Many cities already have this. The runoff from my street flows right to the Bay, and there are signs reminding me of this so I don't pour motor oil into the storm sewer like a dork.

There is an issue of unavoidable contamination of runoff water (street runoff, etc.) that may need to be addressed.

As far as separate water supplies, there are too many opportunities for people in homes to drink water from a hose or something, and municipalities can't risk lower quality water where someone might drink it. For commercial applications, like installed sprinklers or things like that, you might be able to work it.

Individuals can set up systems on their own to use roof rain runoff for "gray" water purposes. Cisterns are a very old concept, and on farms like where my Dad lived as a kid, they often even drank it.