Trust two netbackup master servers
When i have done this I try and use Hosts files on the appliances to ensure good communication over the correct network.
During AIR they communicate directly with each other (Media Server to Media Server) so that is all the comms you need to be sure of.
Next you go to the CLISH of each appliance and do the following:
Appliance
ShowDedupPassword
Make a note of it (do this on each appliance)
Next in the NBU Admin Console go to Credentials - Storage Servers and open up the properties of your appliance. On the Replication tab put the name of the other appliance in, username root and password as per what that appliances de-dupe password was.
Same for the other site if you need to AIR both ways.
Once that is done it is just down to creating the SLP (Same name, one using replicatation and one using Import)
Hope this helps - as long as the communications are good it works a treat!