The problem with partitioning a library is exactly what parts of the library can be partitioned some libraries cannot split out everything that backup products might need to individually own or control whilst other libraries (usually the more expensive kinds) do provide enough facility. Basically there are at least 3 areas that may need to be uniquely assigned to one backup product and not shared.
1) Access to the 'arm' that moves tapes from slots to drives (often referred to as the library itself). Most libraries have one physical arm but some can provide a virtual respresentation (similar to a partition) of two arms that can then be used separately by backup products without either product being aware of the other and with the library itself handling contention requests.
2) Access to individual drives. If you have multiple drives you can partition drives to different servers (as long as the library supports it) and therefore different applications as long as they are on different servers. You can't partition with multiple, apps on one server as this kind of split is at an OS/HBA level and not the application specific level.)
3) Access to mailslots (Import/Export slots). I am not sure if these can be shared or also have to be unique to each application
4) Partitioning the storage slots (this should kind of go hand in hand with the drive partitioning as it would not be that useful to only be able to split the drives and not the storage
Now in most cases Point 1 above is the problem as unique / controlling / owning access to the arm would be required by both backup products and this is where you need to look at the facilities offered by the library as to whether such a 'virtual' split of the arm is possible and can be linked with the partitioning of the drives/storage slots
