$19.80 goes to one artist, and $0.20 to the other. 99:1.
edit: Maybe what you're getting at is that the person who listened to only one song should pay 99x less. Then it really would be pay-per-play. But Spotify is a subscription service. What else should they do with my extra $9.90? Send it all to the one artist? That would be interesting... but then what if I don't listen to any songs in the month? Bank it and send it to the spread for the following month?