Working as a cloud manager, MultCloud enables you to transfer files from one cloud to another cloud or other accounts from the same provider with only one-time login. In this case, it will be a piece of cake for you to copy files from one S3 bucket to another. What you need to do is just adding all Amazon S3 accounts in use into MultCloud (about S3 key) then you can copy data from one s3 bucket to another by dragging/using copy options from contextmenu/employing the transfer service. The last way also offers schedule settings, which will transfer files from one s3 bucket to another even after power off.
*Update: MultCloud added a special Cloud Sync service now, you can use it to do sync cloud drive task directly. MultCloud provides 8 normal sync ways.