Users access the primary path on a standard pair via a network share (\\servername\sharename). They see a merged view of the data on both paths as if the files are all stored on a single device.
The native access control of the underlying file systems determines the user access to the data. All access to the secondary path is made via DynamicFS as if the data were located on the primary path. DynamicFS does not need to relocate the data to give the user access to data on the secondary path.
To set ACLs and attributes on files and folders, you should access the merged view of the data via the network share on the primary path, then set the access control for files and folders as you normally would. If direct access to the path is necessary to set ACLs, make the changes to the folder instance that resides on the primary path.
In a merged view, the users are not aware that the .mp3 files are now located on a different disk than their other files. There is no performance impact for the user.