No solution, but a bit of information. Snapcast appears to be the way forward for simultaneous, multi-device streaming. It is in the plan, but is not yet implemented in Volumio. You can follow the thread on the forum and set it up yourself for playing local music (NAS, USB) and I think it worked (but can’t remember for certain) for radio streams.
Now, Spotify is more difficult. One poster spent some quite some time on this and didn’t suceed (have a search if you haven’t read the post). It is reported on Github that snapcast will work with the ‘librespot’ library (as used by Balbuze in his Spotify Connect2 plugin), but it is not something that I have had time to try for myself (nor have I seen it reported in the forums).
I don’t have any knowledge of Shairport or UPnP via Snapcast.
Hopefully, this answers a few questions for you