Ok, lots of e-mails on this topic with speculation, etc...here's my experience.
SUMMARY: I like having access to drives in order to do testing without having
         to temporarily cable up some drives, BUT there are TSM issues and AIX
issues.

Issues: TSM polls a drive that's in use after an initial request for 10 minutes,
 then makes it
        unavailable. AIX or Atape driver generates AIX errors when one system
attempts access
        when drive is in use by the other.

Environment:
System A- AIX 4.3.2, TSM 3.7.2, atldd.driver 4.0.7.0, Atape.driver 5.0.2.0, 3494
 (lan connected
   to system A), 2 3590 B1A->Ultras models daisy chained to 1 scsi wide
differential card (port 1 connections)
System B- AIX 4.3.2, ADSM 3.1.2.42, atldd.driver (same), Atape (same), 3494
(same), 12 3590 B1A->Ultras
    connected to 6 cards (2 Ultra SCSI cards, 4 scsi wide differential cards)
(all port 0 connections)
* 3590 microcode level B8C2 or D0IB_8C2.fmrz
* 2 of the 12 3590 tape drives of system B, are shared with system A

My experience:
* Heard of sharing 3590s across multiple systems and wished to connect a test
TSM to 3590s and share 2 drives
  between test system and production system.
* Cabled up drives and had a CE configure (a CE function) the autoshare option,
ran cfgmgr etc..
(On this topic, I opened up an ETR/PMR or two...not enough doc..found doc in
hardware
reference manual (online), but no reference in the operator guide, nor intro &
planning guide).
http://www.storage.ibm.com/hardsoft/tape/pubs/pubs3590.html
How a CE can configure...and referencing doc:
Get to the CE panel by pressing the Wrench, then select proceed ->
Config/install ->
drive options -> scsi config. Here you have the option for Autoshare
Enabled/Disabled.

| Autoshare Enabled/Disabled Allows the CE to enable the Autoshare option
for | host application.  The default for this option is Disabled.

The above is from the manual "IBM 3590 Tape Subsystem IBM Maintenance
Information Models B11, B1A, E11 and E1A"
Pub Number#SA37-0301-0

* Configured drives to ADSM and TSM.
* Tested access from systems to 3590 drives (not at the same time),
successfully...no issues.
* Attempted access to 3590 drive from system A, while system B had tape drive
and found
  that TSM will detect that drive is unavilable, TSM will then poll drive and if
 drives
  is still unavailable TSM will make the drive unavailble.
  AIX or Atape.driver under this scenario will  throw multiple TAPE_ERR4 errors.

NOW, the meat. TSM 3.7.3 (available this summer (U.S. summer..June maybe)) will
have 'better' code for this and will
'continously' poll the drive until it becomes back available. (I believe/hope
that they will still make truely broken drives
unavailable, though...). So, I closed my pmr/etr within the ADSM/TSM arena.
Manually, updating drives online and offline
while having them cabled up is better than temporarily cabling up drives and
loosing access to other systems (FOR NOW).

The other meat...I have an etr/pmr open within the AIX/3590 microcode area or
arena... in hopes of making sure that when
TSM 3.7.3 has 'better' support....I won't be getting tape_err4 errors when one
system has a drive and another system attempts
access to that same drive.

Recomendations: In a TEST & Production environment (not 2 production
environments), sharing of tape drives can be pulled off..."manually."
I just make the tape drive unavailable/offline to one TSM/ADSM and make it
available/online to the other. This allows me to test, whenever,
without having to make hardware/cabling changes.


* Oh, by the way... for non TSM levels, you, by default, have 3494shared
yes....the 3494shared option was 'built' to give a customer(s) the option
to NOT interrogate the availability of a drive....ie 3494shared NO was really
the code/parm/option that was recently made available...and again...you
have 3494shared yes...

Still new 'stuff' here....and it seems that ducks are still in need of
alignment...(are these ducks really in a row....?).
One more reference point: http://www.redbooks.ibm.com./solutions/tivoli/
see Tivoli Storage Manager Version 3.7: Technical Guide (SG24-5477)
7.5.2 3494 tape library sharing with twin tail SCSI connected 3590
argh!
Please excuse the length of this item. I made every attempt to keep it clear and
 accurate with brevity in mind.
Thanks Tim Williams Frito-Lay/PBSG  (I hope "you'all" find this helpful).