I don't think doing incremental 'forever' backups will be what you want.. this would be painstakingly slow when restoring. You could consider the use of synthetic fulls but still...
I don't know if any legal requirements apply on your situation (depends on the kind of business you work for and the regulations that apply) but have you thought about the following ?
- Perform your backups at site 1 to a Maglib (deduplicated, although it doesn't have to be)
- Use CDR/SDR/.. (Commvault's replication) to site2 to create a read-only replicated maglib
That way you're backing up on site1 at LAN speeds, and still have a "asynchronous" replica of your data on site2 in case of a disaster. Granted, you are using twice the space for backups (site 1 and site 2)
In order to avoid the initial transfer of all the first backups, you could place the replica maglib Media Agent at site 1 first and perform the initial replication at LAN speed.. Once you're finished you could ship that MA off to another site and re-established the replication again. (you'd have to mind about IP/hostname/routing etc)
If you do want to stick to your original scheme, you may want to wait until Commvault starts with true client-side deduplication someday ...