Configuring AIR between two NBU domains using NetBackup Appliance 5230

You are correct, Felix.  What you're referring to is cross-replication, where you backup images in both sites and then replicate them to each other. 

As GHamilton points out, you need to make sure you're sized on both sides for the amount of data you're going to be storing.  I have also run into situations where customers didn't size properly for that and it creates some sticky situations where they have to make hard choices between retention and replication until they can get more disk space.

I will point out that it is not REQUIRED to use a trusted master setup.  It makes things more flexible, but you can do this between versions of NBU that do not support trusted master settings, for example.