It looks like Mac-Sync will do the job you are requesting.
I downloaded and paid for it and I am not sure how it works?
I added the 2 external hard drives and hit "sync" and it said "complete" 5 seconds later but none of the actual files had transferred over between the hard drives?
See image attached
If you open RTB_ARCH_A, do you show files there? When you added the connections, did you select to sync subfolders?