I learned that the Hifiberry OS and Sonos can play simultaneous on iTunes with Airplay.
It has to be through OSX iTunes though. I tried using the MIDI app to make an aggregate device, and it doesn't work. I also can't get multi-device to work from the tablet iOS which I don't understand. If anyone knows how to use multi-device Airplay outside of OSX iTunes, I'd love to learn!
My partner got the Sonos after their streaming research, and I got the Hifiberry DAC after mine. We have an interesting marriage ;) So I'm happy that this odd couple can play together.
The berry is streaming over wifi at the indoor stereo, and a portable wifi sonos goes outside or around the house as needed.
My iTunes library is small, but I can play streaming files on the laptop. My favorite is WFMU Rock and Soul Radio!