Keeping a external harddrive's copy of my iTunes library in sync

Hi,

I have two MacBooks: one is to surf and fun (we can call this one A) and the other is for my work (this could be called B).

On A I have my iTunes library on the internal HD. I have copied this iTunes library onto an external HD. I've configured B so it's iTunes library is the copy on the external, so I am only able to play my tracks on B when the external HD plugged in. I'm not interested in having the library lying on B's internal HD. This has all worked fine, except that now I've gradually added new files in A's library, which does not exist on the external harddrive and indirectly nor on B.

How do keep my library from A sync'ed and up to date with the copy on the external?

Cheers,

L