We use one network to represent EXISTING storm structures and pipes. We routinely need to connect a proposed pipe network to one of the structures within the existing network. I already have a request to be able to create the existing network via the survey tools (https://forums.autodesk.com/t5/civil-3d-ideas/import-survey-create-pipe-network/idi-p/12493576). When using that feature, the existing network would be included in the survey file and attached via data shortcut to the proposed design file.
I'm mainly talking about gravity networks but this would also be useful for pressure networks.