Hello,
the Content Store Selector would be one way to do this, but it is only available in the Enterprise version of Alfresco. With it, you could define a behavior that automatically assigns the ContentStoreSelector aspect to renditions with the name of the alternative store to use and Alfresco would use that store to persist the content.
In Alfresco Community you are basically forced to reimplement the functionality provided by the Content Store Selector with a own ContentStore.
Regards