Let me clarify... replicate as a term in NetBackup refers to the INTER domain replication of images. That means that there is 2 Master Servers (2 NetBackup domains) and backup images are replicated between them using the OST feature and an OST enabled storage device (like DataDomain). This process also doesn't require you to configure replication on the DD, only associations between the LSU's configured on the DD.
If you don't have another Master Server you can add the DataDomain to the same NetBackup Domain (and Master Server) and then duplicate the images from one DD to another.
This type of configuration is obviously not the same as the mentioned above as there is only one Master Server and both DD's depend on it. If the master server fails then both DDs are rendered unusable as the master server contains the catalog.
If it's just a theoretical question from "what if" series then it's theoretically possible. If you want to protect your backup environment better but you don't have DR-site then you can add second DD into current NBU domain and use optimized duplication. If DD is the only storage devices you have then DR-procedure will be not very easy (actually it depends on some factors but might be tricky).