Couldn't the path "C:\Documents and Settings\User\Application Data\J River\Media Center 12\Library" be synced just like the media files to keep the source and target MC libraries in sync? Seems any changes on either source or target would change the date of the file and sync the newer one to the older one. This would be more automatic than a backup and restore but will MC choke on this because the PC names are different? Of course MC could not be running on either.
This is fine if you do it from one PC to another. You can't create a mix of two libraries. You must decide which PC is the source and duplicate everything on the target PC.
The PC name is not checked at any stage when a library is loaded or used. Just make sure that SyncBack creates identical media file folder paths.
Also, it doesn't matter if the user name and thus the default library location on the PCs happens to be different. MC doesn't care about the location of the library files after a library is loaded. Only the media files must be in identical locations.
You can also add the thumbnail folder to the file duplication process if you want to avoid rebuilding the thumbnails after each sync. (MC uses a cache system for album art. You can find also the thumbnail folder under C:\Documents and Settings\[User]\Application Data\J River\Media Center 12\... )
C:\Documents and Settings\[User]\Application Data\J River\Media Center 12\Library
I have never liked the default Windows path structures. Personally, I always try to use the shortest possible paths. My library files are in C:\MC12lib\ and for example my main audio storage location is the very root of an X: drive. (I use the rule: X:\[Album Artist (auto)]\[Album]\[Disc #] - [track #] - [Name])
I don't have a physical X: drive or partition, but I map a shared base folder with that drive letter on each PC on our home LAN. In this way I can use any folder as a drive X: and use an identical MC library on each PC. That folder can be mapped over LAN or it can be on a local HD as well.
When I am travelling the same library works on my laptop. I just map a shared folder on a big external USB drive as X:.
(This is a bit simplified version of my actual system. I have also the virtual drives Y: and Z: and I use locally the subst command instead of mapping shared folders and I have added some additional storage locations under the same virtual drive letters with NTFS junctions, but that stuff is not needed for making the X: drive system to work.)