The 3590 is a dual ported device.  It actually has two fiber port WWPNs.
AIX actually is talking to both ports all the time but only sends out
primary IO right now to whichever is the current primary.  What is coming is
load balancing based on what I know.  This way the ES-1000s will have
balanced IO if both ports are connected to 2 different pathways (ES-1000s).
Finally, there is a reason 3590 is far superior.