The Rhythmbox Library Database is stored in ~/.local/share/ryhtmbox/rythmdb.xml by default but you can invoke rythmbox this way to take any file you want:
This way you can create two diferent launchers that target to two different music libs. This is terrific but I found just one annoying behaivior. If you have the auto-inspect option enabled pointing to /home/myUser/Music and invoke the launcher that use the LowQuality folder when you include new music files to /Music they will be included in the wrong lib. This is a little difficult to explain :3
The easiest solution that I found was changing the path to the inpection folder before calling the launcher.
you can invoke rythmbox this way to take any file you want:
gconftool -s /apps/rhythmbox/library_locations -t list --list-type string [file:///home/myUser/MusicLowQuality] && rhythmbox --rhythmdb-file=/home/myuser/.local/share/rhythmbox/lowqualitydb.xml
gconftool -s /apps/rhythmbox/library_locations -t list --list-type string [file:///home/myUser/Music] && rhythmbox --rhythmdb-file=/home/myuser/.local/share/rhythmbox/normaldb.xml
I hope this can be useful for someone :)