2 Denon Receivers - Combine into one Media_Player?

Finally figured out how to start playing Spotify on a specific device, but now I need to play it simultaneously on 2 devices. Specifically, 2 identical Denon AVR Receivers. One is for my theater setup, and the other provides audio to speakers in the ceiling around the rest of the house.

Here’s my working script that starts playing Spotify on the receiver. What am I missing?

  alias: Pop Playlist

    -  service: media_player.select_source
         entity_id: media_player.spotify_xxxx
         source: "Denon House Audio"

    -  service: media_player.join
         entity_id: media_player.spotify_xxxx
         group_members: media_player.basement_theater

    - service: media_player.play_media
        entity_id: media_player.spotify_zach
        media_content_id: spotify:playlist:xxxxxxxxxxxxxxxxxxxxxxxxxxxx
        media_content_type: playlist

My script will only send audio to the Denon receiver for the house and the Denon receiver that powers the theater never gets any love. Where I’m confused is the source listed as “Denon House Audio” is the name that appears in the Integrations for the receiver itself - it does not reflect the name of the actual zones.

The theater receiver is called “Basement Theater”. You can’t merge sources, but you can merge media_players, and I can’t seem to get the script to accept a media_player name in place of “Denon House Audio”, so there’s nothing to join.

Would love some help. Thoughts?