How to output sound on 3.5 mm stereo jack and via UPnP-stream simultaneously?

Imagine a standard desktop computer. Connected to this computer is a pair of loudspeakers via common 3.5 mm stereo jack (AUX). Everything works fine so far. ---------- Now we have a further speaker in another room of the building that does support receiving of UPnP-streams. How can I configure a Debian-based system to keep emitting the sound on the classic stereo jack **but additionally also stream it via** UPnP, so I can feed both speakers simultaneously?