I like to have iTunes in full screen and have Airplay volume controls on the main screen. However with iTunes 11 you no longer get a separate volume control window. Just by accident I discovered that you can have the full function Mini player and a full screen (or Windowed) "copy" of 11 running at the same time. While in Mini mode in Safari click on an App at iTunes.apple.com to view in iTunes. It will open a separate copy of iTunes while leaving your Mini play running. Both have full control over iTunes.