Single Song Folder
On Windows
You can have one song folder for different FoF installations under Windows, if both of them are installed on NTFS partitions (or on one NTFS partition). Try using Junction tool, just be sure to read the article first.
Run cmd first, go into the directory with junction.exe and type the following:
junction
Click Accept.
junction c:\folder\you\want\your\songs\to\be\in d:\folder\with\your\songs
Do not reverse the order of directories.
On Linux
The instruction on doing the same thing on Linux is similar to using junction on Windows.
In console, log in as root and type:
ln -s -d /folder/with/your/songs /folder/you/want/your/songs/to/be/in
Do not reverse the order of directories.