If one is just a copy of the other and the two Macs
are authorized under the same Apple ID, you should
be able to just use a file sync app to keep both identical.
Although a different scenario, I have two hard drives attached
to my Airport Extreme Base Station, each with my iTunes folders.
One is actually my prime and the other is a straight backup.
All my Macs are set up to use the prime when starting iTunes.
They all can also open the backup as well. All I do is use my
sync app to do a straight backup and works fine. I don't see why
this should not work using a true two way sync.
Remember that the sync/backup needs to encompass all that is in
the iTunes folder.