Just to confirm... the receiver is a Yamaha RX-V685?
If so, then it's 100% possible.
You can actually use MusicCast to share any source from Zone 1 to Zone 2 (except for Airplay and DSD audio files.
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I would hazard a guess that you have the 'power amp assign' setup incorrectly and aren't actually sending audio to the Zone 2 speakers, instead you're just playing radio through the 'surround rear' speakers when in a 7-channel stereo setting (or similar).
Step 1 : Make sure you setup the Power Amp Assign correctly. You can use the receivers on-screen GUI to do this.
Step 2: Download the MusicCast app and make sure the product is linked as per the app's instructions.
Step 3: On the MusicCast app, turn on 'Zone 2' in the settings. A separate room will then show up that shows you the sources that can be independently selected in Zone 2. This makes it super simple to see what is possible.
Step 4: Start playing around with the 'link button' and see what you can share from zone to zone... the beauty with MusicCast is that you can share HDMI / optical / RCA / coax / USB / DLAN / Spotify / Tidal etc etc... so most of your needs should be sorted.