We share a 3494 with a mainframe vts and a aix/rs6k based tsm server.
The vts origionally had 6 frame units with 6 drives (base end unit,
high availability frame at the other end, and 4 storage frames with
drives).  To support TSM we've now added 8 more frames and 12 drives.

It took the IBM folks a little work to get the library manager set up
to allow dual connections, but once done it's worked very well.
Since each system has it's own dedicated tape drives performance isn't
a problem.  Our vts doesn't access the robot for mounts very often,
so robot access contention isn't a problem.

The only problem we have had is coordinating outages with with the
mainframe folks when we needed the new frames added. Adding frames
to a 3494 NOT a fast process - the last set of 4 frames we installed
for TSM required an 8 hour outage.