My understanding of these is that each machine will play anything from the storage it can see. So if you networked two together then two tracks can play simultaneously from the 'pooled' HDD's. The same would apply to adding a NAS with compatible music files on it.

I'm currently working on a prposal with four of the Opus HDS500's in it, which is the same but with better control options (and not available yet!). Looks like a good unit for the price too.

You can also add a second HDD internally if you want.