I symlink (NTFS mklink) all of the config directories for each account together per instance of WoW. For example:

C:\wow1\WTF\Account\account1 => normal directory
C:\wow1\WTF\Account\account2 => symlink to ..\account1
C:\wow1\WTF\Account\account3 => symlink to ..\account1
...

C:\wow2\WTF\Account\account1 => normal directory
C:\wow2\WTF\Account\account2 => symlink to ..\account1
C:\wow2\WTF\Account\account3 => symlink to ..\account1
...

If you do it this way each instance of WoW acts like all of your toons share a common account. Since the toon names have to be different anyway, there aren't any settings collisions.

I don't try to share settings *between* instances of WoW. I found that causes problems when you log off -- changes from toon1 stomp those from toon2, etc.