how to keep two iMac's in different locations completely synchronized I alternatively live in two different places, a few month in Europe and then a few months in Asia.
In both places I have an iMac with OSX version 10.8.5

I started by copying the complete content of one of the iMacs to the other one; so at that moment they were identical.
Now I will be in Europe for some time and work on my machine there. 

Once I move back to Asia, what can I do to make my iMac there identical to the one I just left in Europe and vice-versa ?
You could probably configure something with DropBox.

Create symbolic links for preference files for your programs, and save all of your documents to the DropBox folder for syncing.

The downside to this is that if you have more than 2GB of data, it will cost money.