If you are referring to SharedDisk option, then yes, they have to be same O/S.
Extract from Whitepaper
http://eval.symantec.com/mktginfo/enterprise/white_papers/b-whitepaper_nbu_disk_based_data_protection_options_4_2008.pdf
3.3.1 What is SharedDisk?
SharedDisk allows LUNs in a disk array to be shared between multiple Media Servers in the same way that tape drives can be shared with the Shared Storage Option. As in AdvancedDisk, the SharedDisk volumes can be defined in a disk pool, however in SharedDisk storage the disks are in a storage array and the disk pool is presented over a SAN connection to multiple Media Servers.
Disks in a SharedDisk disk pool are not presented to multiple Media Servers simultaneously; instead they are dynamically assigned to individual Media Servers when required. When a disk from the pool is assigned to a Media Server, the file system is mounted, and the backup image is then read or written. Once the backup or restore operation has completed the disk is dismounted and returned to the pool. Any disk within the disk pool may be presented to any Media Server at any time. Since a disk can be mounted on different Media Servers, the
Media Servers participating in the pool must be the same platform type running the same operating system.
On page 11 & 12 of this whitepaper is a table that summarizes the advantages of intelligent disk devices over VTLs.
Also see this article regarding NBU disk options:
https://www-secure.symantec.com/connect/articles/whitepaper-netbackup-disk-based-data-protection-options