I think what you want to do is grow your pddo volume, not add a second. Per the Compatability Matrix:
ftp://exftpp.symantec.com/pub/support/products/NetBackup_PureDisk_Remote_Office_Edition/332221.pdf
You can have up to 16 TB of Back End Storage per physical server, and up to 100 TB in a single Storage Pool ( ~6-7 Content Routers fully populated)