I made some investigation and went through the sources in android 4.x.
Seems as if they hard-linked the two streams "ring" and "notification". Options for using separate values have been removed from the sources!
As I was confronted by some users of my app "EasyProfiles" complaining about this fact, I decided to integrate a "workaround" solution.
Although I do not like to make advertisment on this platform I found that you might be interested in this solution.