There are other reasons to consider dual water distribution systems, beyond the idea from the previous post that such systems conserve the best sources of fresh water for potable use.
There can be substantial deterioration of water quality as it travels, sometimes for days, from the point of production to the point of use. The reason this occurs is that the insides of these pipes can be coated with all manner of biological and other growths, which degrade water quality. Water travels very slowly through distribution pipes because the pipes are sized extra large for producing a lot of water quickly in the event of a fire. But fires are rare, so water can basically stagnate in a distribution system. A separate, small diameter piping system for potable water would have far faster flow rates, resulting in far less water quality deterioration than a conventional system.
It costs a lot of money to produce high quality potable water, regardless of the raw water source, and we need to preserve its quality to the extent possible. Dual systems are one way to do that.