It’s not clear why you need to run two instances of Firefly. Firefly can be configured to serve files from multiple locations – just use the “add folder” facility on the web-based config page, for example.
The main purpose in running more than one instance of Firefly is that it allows the SoundBridge to connect to and then browse or search more than one selection of music, perhaps for different user’s who only want to be able to see ‘their’ music. Where the music is held is irrelevent – it could all be on a single disk or on more than one.
But to answer your question – this thread may help.
There are a couple of messages in there which while not a complete step-by-step guide, should allow you to figure out what’s needed.
Hope that helps,