Its really up to the needs and requirement of the organization.
The following option are available:
Option 1 - At the moment since you have a Global DDB in the Cloud, you can create the Secondary Copy for your Local Storage Policy, and point it to the Cloud Global DDB, so you can take advantage of the Deduped Saving. (using the same DDB and Cloud Library)
Option 2 - you can create new Cloud Library and Cloud DDB to manage the Secondary Copy. (using the different DDB and different Cloud Library)
Both options work, and it really comes down to which option is more suitable.
Hopefully that helps