This article explains how to easily make AWS cross-account backups with CloudRanger.
Copy your backups to additional AWS accounts
A backup policy will automatically generate a backup in the origin AWS region. If you require a cross-account backup to another of your AWS accounts, we provide the functionality to do this.
When you choose to create or edit a backup policy from the main backup policy page, simply select the Save an extra copy to another account checkbox and select the preferred AWS account and region to save the additional backup copy. This will then save the additional copy in your chosen account and the retention of these snapshots in your additional account will be the same as outlined in your backup policy.