But will ATV drive TWO airplay speakers like iTunes does?
I kind of assumed Airplay 2 would enable this, but it doesn't seem to be. Maybe when AirPod arrives they will support this? For now, ATV Audio Output seems to be an either\or proposition.
I have 2 ATV4 (connected to Home Theater and soundbar via optical from TV), and AP Express (connected by AUX to a portable speaker). When I want whole home audio, I fire up iTunes on my Mac (connected via Optical Audio to an old Home Theater system in the office). I then launch the old Remote App (the one with the blue icon, not the new black icon Remote app) which sees all of the airplay devices and can control music in iTunes. I select a playlist, enable output to Mac + ATVs + AP Express and the sound streams to all of my devices. The sound sync is flawless and volume can be controlled on all outputs using the Remote app.
The AP Express is setup in WiFi client mode, with Airplay enabled. The ATV are both hard wired to my network. The nice thing with this setup is the portable speaker can be placed anywhere in WiFi range to provide sound in that room (kitchen, patio, hot tub, etc).
I don't have an Apple Music subscription, but assume I could play that with this setup as well. For now, my 5,600+ songs provide enough variety to play for 18 days without repeating. I have a few playlists with nearly everything in it and set that to shuffle mode and it is a unique experience each time.
The nice thing with this setup is, no additional cost. AP Express was sitting around gathering dust until I set all of this up. This also works with iTunes on PC. AP Express can be picked up for $49 (Refurb) from the Apple online store (when available) or less on eBay. Older 802.11g Express can be found for under $20 on eBay. The 54Mbps of the older G model is more than enough for audio streams, that is the model I use.