Why doesn't the duo subscription allow two simultaneous sessions regardless of the account?
This restriction only creates problems when using a third party system to listen to the music (sonos, Amazon, Google, ...).
Like many others when my wife launches a playlist from the sonos system it cuts off the music at home. And the fact that I added her account on the sonos system doesn't change anything. Sonos by default uses the first account registered.
And no, I don't necessarily use the Spotify app. I use the sonos app, my KNX touchscreen, Google assistant...
Just like Netflix, the subscription defines the number of simultaneous sessions without consideration of unnecessary constraints.