Another option might be to proportion revenue in accordance with membership sources into the cartel. E.g. if site 1 delivers 10 new subscribers, and site 2 brings in 2 new subscribers, the money goes to the sites in a 5:1 ratio, minus maybe 10% off the top for operating expenses.
The problem I see is that with spotify it's pretty easy to determine the rate of content consumption (e.g. songs or minutes listened). News sites are pretty adept at turning one piece of content into many (multi-page articles or slideshows), and the system would have to protect against that.