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.