To me, it sounds like you need to clarify your requirements and come up with a coherent definition of what "separate" means in practice according to whatever requirements you may fall under. It's all a matter of perspective. I'm not being facetious; in a very real way my laptop is connected to Symantec's servers right now in the course of my posting this. They aren't separate in an absolute sense. No technology is separate in such a sense without an air-gap. If you want "separate" in an absolute sense you run it the same way (at least some - that I'm familiar with) jails run their key systems - on an isolated network disconnected from the rest of the network and the outside world. Data in or out needs to be manually transferred with drives.
For example, suppose someone's operating in a multitenant vmware model whereby the underlying infrastructure (vmware host management, vcenter, netbackup, storage management, etc.) is logically separated (separate ip address space, separate vlans, a firewall between them, etc.) from the tenant/guest servers. Suppose also that the scenario would involve hypervisor-based backups of said guest servers. What's stopping someone from defining netbackup policies on a per-tenant basis, creating different volume pools for the tapes, having separate sets of tape labels (per-tenant), constructing appropriate tape rules to maneuver the tapes into the appropriate destinations, and backing up from there? With suitable documentation, procedures, and policies in such a situation, I would think one would be able to demonstrate that the data is going to the right places and not being mixed inappropriately, without much in the way of practical risk to the tenants data integrity/separation. That's a logical approach though. You may be subject to some regulatory scenario, or technical obstacle, depending on the nature of your data and environment, that makes such an approach impractical.
Again, ignore the technology. This isn't about NetBackup, VMware, any of it. Start with requirements. If you use the specific name of a product in the conversation you aren't talking about requirements, you're talking about solutions that meet those requirements.
"I need to use the same tape library across all the backups" - that's a requirement you can use for these purposes.
"Different customers data needs to end up on different tapes" - that's a useful requirement as well.
Licensing questions about netbackup aren't requirements though. Those are implementation specifics.