This will get to be a muddy solution. Your best bet is to use one as the source and have the other 'spoof' it.
I would propose loading it all up in SharePoint, and then attach to the library as a network drive.
If you wanted to keep it all on the network drive, you could use a 3rd party solution to browse it in SP, or crawl it with search and customize a search web part.
If you reaaalllyy want a sync scenario, you might be able to use SkyDrive Pro, which treats libraries like DropBox does. You'll need a computer always running to monitor and handle the sync.