They don't have COMPLETE and PERFECT separation of storm and sanitary sewers, but they are substantially separate systems at this time almost everywhere. They just have a finite capacity, and the overflow often ends up mixing in older cities during storms or "floods" (defined tautologically).

The cost of retrofitting an existing city aside, the sanitary sewer is a subgrade utility tunnel, by design and by cost footprint. If you're already digging a big ditch and installing infrastructure there, it doesn't cost much more to have space for other utilities. We're not talking about building a basement for the entire roadway, we're talking about dropping modest size pipes under the sidewalks (or in many places, the lack of sidewalks) and enabling access through manholes.

> We're not talking about building a basement for the entire roadway

We are talking about a basement for the entire roadway if "[m]aintenance crews can just drive through in cars and do their jobs, without stopping traffic and digging out pipes," like the GGP was talking about. Also, that's what all the pictures in the previously linked https://en.wikipedia.org/wiki/Utility_tunnel look like.

> ...we're talking about dropping modest size pipes under the sidewalks (or in many places, the lack of sidewalks) and enabling access through manholes.

I think you have a different idea, which sounds like conduit or something between conduit and a full tunnel.

  > substantially separate systems at this time almost everywhere