you can use both veritas and nt backup with same media device. But it is not a recommened way.
Better to configure seperate devices if you are necessary to use both. If you use both, before using the tape device with NTBackup, ensure that all Backup Exec services are stopped. You also have to make sure that the removable storage management service is running.
Also the veritas drivers for the tape drive must also be uninstalled.
For more inforamtion refer,
Hope this helps