NBU Device Configuration

Have you checked compatibility for your tape library in HCL http://www.symantec.com/docs/TECH76495 ? 

Only IBM robots are supported with AIX.
See this extract from HCL:

2. On IBM AIX, use of the IBM Atape driver is required with IBM tape libraries. The Atape driver's multi-path feature must be disabled.
3. NetBackup has been supporting non-IBM tape libraries (robotics) with an IBM AIX robotic control host using the "ovpass" driver for many years. Changes and new functionality in AIX 6.1 and 7.1 have resulted in some issues with ovpass and an inability to support new functionality such as dynamic tracking and controlling SAS robots. Therefore, beginning with NetBackup 7.5, Symantec is no longer supporting the ovpass driver.  Existing customers may continue to use ovpass as they are today, but if issues are encountered Symantec will not be able to resolve them.
Symantec recommends using another OS platform as the robotic control host.

Only one media server can be robot control host - so, robot can only be added on one of your media servers.
In a shared environment (with SSO license), all tape drives can be added to all media servers.

PS:

NBU cannot be clustered on media servers.
Each cluster node must be installed as independent media server.
See :
NetBackup in Highly Available Environments Administrator's Guide :  http://www.symantec.com/docs/DOC5183 

More reading matter:
Read the AIX chapter in  NetBackup Device Configuration Guide:  http://www.symantec.com/docs/DOC5186