Why would anyone want to do that?
Why don't they just run smaller septic sewage pipes inside the larger existing storm sewers, and then reroute them appropriately at the treatment plant end?
Or is that too simple, and something that the worthless unions would oppose?
Because of plugging.
It reduces the capacity of the storm drains, which are often inadequate in older cities. Also, they have to do a connection at every house and business. Where I used to live that would be every 40 feet they would have to dig down (usually through the street pavement), enter the storm drain, connect to the newly installed inner sanitary drain. Then seal it up and make above ground repairs as needed.
The latest of many giant storage tunnel systems going into Cleveland (200 feet down) and other cities requires NO above ground disturbances of yards, sidewalks, paving, or traffic interruption other than four shafts at the beginning, end, and two other places. No one will have any idea what is going on below them. In a few years, no more turds in the lake.
Another advantage is that the storm sewer system will have a much larger capacity leading to very little street flooding in massive downpours.
Not a perfect solution but a very good one nonetheless.