Until recently, the instructions in our FAQ regarding syncing books
http://www.kindleboards.com/index.php/topic,73465.msg1183688.html#msg1183688 were about the best you could do.
However, Amazon recently changed the way personal documents were handled, so it may be that if you send the books to your Kindle "cloud" by emailing them to your free.kindle.com account, they will sync between the Kindles.
I've only got one Kindle, and they haven't added the functionality to the Kindle for PC/Android etc apps yet, so I can't test it.

You won't be able to do it if you transfer the books onto your Kindle(s) any other way, eg by cable or by using Calibre's built in website - it relies on the Amazon servers to do the sync.
Experiment with it and let us know how you get on!
