There’s an updated (January 2020) Power Apps Licensing Guide. There’s some useful information added on page 3 detailing the licensing requirements for external users, and confirmation included on page 4 that if the same Power Apps app exists in two environments, a user would require two Per App SLs to access both. As usual there’s a detailed Change Log on page 25, and you can find the updated guide here: http://bit.ly/PowAppsLGJan2020.