I am sorry to say, but there is no way to actually "mirror" your two macs to the best of my knowledge. However, there are some options out there that will come close to suit your needs.
With dropbox, you can set certain folders to sync to your dropbox account. So if you wanted your documents synced between the two macs, you would sync your document folders to your dropbox account on both computers, and all documents would be combined. Whenever you add a new document, or change a document, it automatically changes in the dropbox foder as well.
Dropbox can be used for any folder. So you are not limited to jsut documents. However, Dropbox comes with a free 2GB, and mroe GBs cost extra.
Another option could be ChronoSync. I have not had personal expiernce with this application, but the reviews I had read, sound very promising. I am not going to go into full detail about this one, but I think its worth a read. http://www.econtechnologies.com/pages/cs/chrono_overview.html
A feature of ChronoSync which will probably benefit you the most: http://www.econtechnologies.com/pages/cs/chrono_tips18.html
Respond back with more questions and concerns about this. I found this a very interresting question and loved answer it. Have a great day and may the odds ever be in your favor. 😉