In relation to Global Deduplication Policy or a normal Deduplication Policy (depending on Service Pack), Commvault has removed the capability of adding additional Data Path, mainly because you will be spreading the Deduped data across different Disk Library (and potentially different backend storage).
The above is not best practice and would not be recommended, because once you add another Disk Library, the associated Mount Path in the Disk Library will be linked to the DDB. Meaning you will have multiple-point of failure, and in the unlikely scenario where you lose one Disk Library (pretty much all your backup might not be restoreable - because the required block might exist in the offline library).
Rather then adding more Disk Library to complicate the setup, it would be recommended to add more Mount Path to the existing Library (recommend to come from same storage) or expand the backend storage (expand the LUN/CIFS/NFS at the storeage level).
Hopefully that helps