You can do so in several ways, as follows:
Use a NetBackup policy to create multiple original backups, assigning the copy to be vaulted to an offsite volume pool in any of the robots. For each robot, configure one vault and one profile that ejects the backups that were assigned to the offsite volume pool in that robot. Only backups on media in the offsite volume pools specified on the Eject tab and that meet the rest of the criteria specified in the profile are ejected.
Use Vault to duplicate images. On the Choose Backups tab of the Profile dialog box, specify the robot to which the profile belongs in the Source Volume Group field. This limits the profile so that it duplicates only backup images that have their primary copy on media in this robot. Specify half of the available drives in the robot as read drives so that an equal number of read and write drives are available. Configure one such vault and profile for each robot.