Today, we actually run an AIX and Windows server to the same library, though
they do use SAN connected drives, it has no bearing on the implementation
because they are two distinct TSM servers not knowing about each other and
not sharing drives.

The issue is more understanding how the 3494 categories work than anything.
Make sure you are using the latest microcode (IBM engineering), drivers
(Magstar FTP site), and patches to TSM.

If you are not sharing any drives, then you just implement 2 separate TSM
servers with different categories for the private and scratch tapes.
Remember that 3494s get the first scratch category and 3590s get that number
plus 1.  So choose your numbers carefully.  I typically chose something like
3500 for the private, 3501 for the scratch, 3502 is automatically assigned
to 3590 when you turn on the parameter that says you have 3590s (recommend
this implementation).

Define both TSM servers to the library.  Create separate Device Classes,
some day you may want to share the drives.  Actually, you could now if you
used both SCSI ports on the drives.  And, I believe at 4.1.3 you could share
those drives with the right microcode level in the drives.

If you want to use a common tape pool this is possible but then you will
have to setup server to server communications and use drive mapping and
sharing.