Are your computers networked together, or are you using an intermediate medium (USB thumbdrive?) to share the libraries?
If you're using a thumbdrive, you might consider setting up Calibre as a portable app, running directly from the thumbdrive, and then simply replicating the library as a backup on both machines (with an autostart or batch file.)
That way your library is backed up in two places, and you're always current without having to remember which library is most recent (and so you don't overwrite the latest library with the older one.)
Even if you lose the thumbdrive, you can just restore from either backup.
If they're networked, can Karen's Replicator do a sync? If so, you should be able to set it up to mutually-mirror the folders, as long as Calibre isn't running on either machine. Set it up as a 4:30AM repeating, automated job -- set it and forget it. (You'd probably have to set both machines to kill Calibre before it ran.)
Another elegant solution is Dropbox. Put your library in your Dropbox folder, and let it do the auto-syncing. Every change is local, but mirrored to the online server, and back to your other machines. Dropbox gives 2 gigs free, and you can social network your way to 8gb for free.