I do not have 5.4 anymore but as I recall the 'Retain Codes' option I think only applies to Marketing Cell codes (CELL########). I think all other codes are supposed to change.
Communicaiton Code isn't replicated based on a 'Save As' to a new Campaign diagram. Nor is it replicated by copying and pasting a Communicaiton Node from one campaign to another. The only way I'm aware that it can be the same as another Communication Code is if it's manually update to be so.
Bottom Line: The only scenario where two different campaigns where each have a Communication Node and must have to the same Communication Node is to update one of them manually to have the same Communication Code as the other.
I'm curious as to why you may be doing this. Are each of these Communcation Nodes in separate channels? Different Schedules? Different Selections?