Such as? Library server sounds like it would be the perfect solution for you. All changes can be synced almost instantaneously. The only 'limitations' of library sync involve destructive changes such as playlist deletion, etc. Per your question, AFAIK, the second "client" that uses a shared library (in your hypothetical) only has read access to the library.
Those are the lmitations I want to avoid, as I will be creating for the first time my library with J River MC. I wan to create and delet playlist, as well as the resto of file management in the desktop which will be faster than from the HTPC.
I went ahead and dowloaded the trial in both PCs (desktop and HTPC) and create a library in the NAS. I was able to open the library from both machines (one at a time) and made some modifications to tags, cover arts, create and delete playlist, then close and opened the library from the other machines and all changes were reflected.
One advantage of this is that I do not need to have both machines on with J River MC loaded as I will need that if I were using Library server.
So far the test were a success. I will give a try to Library Server tomorrrow before I make the decision to buy a license.
Thanks so far for your answer