If you just need to keep two computers in sync, you can use Dropbox (see my sig) -- the important thing to keep in mind is, NEVER let Dropbox sync while calibre is open (they will fight it out to determine who owns the files) and
always sync both before and after using calibre (to prevent editing-on-both-computers clashes
).
Or use Bittorrent Sync to create a slave copy on another computer -- the master computer will overwrite all changes on the second computer.