This can easily be achieved by using the dummy-ups driver. The port field acts as the reference to the "other" UPS served by another NUT server.

Example with dummy-ups driver:

[proxy]
        driver = dummy-ups
        port = upsname@ip-or-hostname[:port]
        desc = "UPS proxy for UPS upsname on server ip-or-hostname"

Also note that there is a clone driver with similar purpose, which allows users to group clients to a particular outlet of a device with a "real" driver running locally, and deal with this output as if it was a normal UPS.

Here the port field references the driver socket name that the "real" UPS driver is using. See its manual page for more details and caveats.

Example with clone driver:

[realups]
        driver = usbhid-ups
        port = auto

[clone-outlet-1]
        driver = clone
        port = usbhid-ups-realups
        load.on = outlet.1.load.on
        load.off = outlet.1.load.off
        load.status = outlet.1.status
        [...]

This allows to group load attached to a separately manageable outlet (or group of outlets) on larger UPS and ePDUs, in order to power those devices on/off together. This may be also useful to delegate management of feeds to devices for purposes like hosting or supporting hardware for smaller teams sharing a rack in a larger company.