Alfresco provide Export and Import feature, you can try to export all the first repository, or a single space, using Export feature on Administration Console in Alfresco Web Client, you must login in Web Client as an administrators.
When you view the target space in Web Client, you must click on Administration Console (the target is the space that you are viewing).
Then you can select the spaces or entire repository to export, and you must select where Alfresco stores the ACP module (export of all of your contents).
Then you can download ACP module and you can try to import in the second repository.
If you would like to solve this problem with an implementation I think you can do it in two way:
1. You can create an Alfresco Webscript/Job that automatically read all contents from the first repositoty and it store them on the second repository via Alfresco Webscript RESTful API.
2. If you would like to use Alfresco Web Services API you can create a J2EE application that call the first repository to real contents and it can store contents calling the second repositoty, all invoking Alfresco via Web Services API.
Hope this helps.
