Up until Alfresco 5.1 there was a content store implementation that allowed you to replicate content to multiple backing stores. This has since been removed since the main use case it served was not the replication during writing, but the aggregation during reading, e.g. look for content on multiple stores and return it if it was found on any of them (i.e. in case you had to move part of your content to another disk as part of archival or when your previous disk ran full). This use case is better served by the aggregating content store implementation.
Since then there is no out-of-the-box feature for writing to two content stores, i.e. two S3 instances. But it should be quite forward to implement a custom content store to replicate / duplicate the write...