ITunes Sever on Synology NAS and Apple Music

Watch Erica Sadun's AirFlick project. It can do what you want from a host running Mac OS X, so one would expect a GUI-less version to appear soon that could be ported to your NAS.

iTunes keeps a lock file in the directory to see if there is an instance of iTunes running - only one is allowed at a time.

Similarly, if you're using the same directory, each version of iTunes will overwrite the library file, so trying to maintain two different sets of music within one tree isn't really possible. At least, that's what happened last time I tried this.

You're better off having Home Sharing enabled, and then having one Macbook at a time run iTunes pointing to the library.

The other can then use a blank dummy library, and play whatever the first is sharing through Home Sharing. Either can run in each role, just hold down Option at start-up to decide if you want to use.

Personally, given a family with a bunch of iPhones, AppleTVs, and laptops, I have a single iTunes server machine that is always on, with a very large library. All music is in my iTunes Match account, all videos can be synced over the air through home sharing, and wireless syncing. It's the closest I've found to an iTunes server device.